Spectral beam combination of laser beams

ABSTRACT

A method and apparatus use a photonic-crystal fiber having a very large core while maintaining a single transverse mode. In some fiber lasers and amplifiers having large cores problems exist related to energy being generated at multiple-modes (i.e., polygamy), and of mode hopping (i.e., promiscuity) due to limited control of energy levels and fluctuations. The problems of multiple-modes and mode hopping result from the use of large-diameter waveguides, and are addressed by the invention. This is especially true in lasers using large amounts of energy (i.e., lasers in the one-megawatt or more range). By using multiple small waveguides in parallel, large amounts of energy can be passed through a laser, but with better control such that the aforementioned problems can be reduced. An additional advantage is that the polarization of the light can be maintained better than by using a single fiber core.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. patent application Ser. No. 11/420,751entitled “MONOLITHIC PUMP BLOCK FOR OPTICAL AMPLIFIERS AND ASSOCIATEDMETHOD” filed on May 27, 2006 (which issued as U.S. Pat. No. 7,941,019on May 10, 2011, which is a divisional of U.S. patent application Ser.No. 11/420,729 entitled “FIBER- OR ROD-BASED OPTICAL SOURCE FEATURING ALARGE-CORE, RARE-EARTH-DOPED PHOTONIC-CRYSTAL DEVICE FOR GENERATION OFHIGH-POWER PULSED RADIATION AND METHOD” filed on May 26, 2006 (whichissued as U.S. Pat. No. 7,391,561 on Jun. 24, 2008), which claimsbenefit of U.S. Provisional Patent Application 60/703,822 filed on Jul.29, 2005, titled “FIBER-BASED OPTICAL SOURCE FEATURING A LARGE-CORE,RARE-EARTH-DOPED PHOTONIC CRYSTAL FIBER FOR GENERATION OF HIGH POWERPULSED RADIATION,” and U.S. Provisional Patent Application 60/746,166filed on May 1, 2006, titled “FIBER- OR ROD-BASED OPTICAL SOURCEFEATURING A LARGE-CORE, RARE-EARTH-DOPED PHOTONIC-CRYSTAL DEVICE FORGENERATION OF HIGH-POWER PULSED RADIATION AND METHOD,” and U.S.Provisional Patent Application 60/797,931 filed on May 5, 2006, titled“FIBER- OR ROD-BASED OPTICAL SOURCE FEATURING A LARGE-CORE,RARE-EARTH-DOPED PHOTONIC-CRYSTAL DEVICE FOR GENERATION OF HIGH-POWERPULSED RADIATION AND METHOD,” which are each hereby incorporated byreference in their entirety. This application is also related to U.S.patent application Ser. No. 11/420,730 entitled “MULTI-SEGMENTPHOTONIC-CRYSTAL-ROD WAVEGUIDES FOR AMPLIFICATION OF HIGH-POWER PULSEDOPTICAL RADIATION AND ASSOCIATED METHOD” filed on May 26, 2006 (whichissued as U.S. Pat. No. 7,430,352 on Sep. 30, 2008), which isincorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under contractFA9451-04-D-0412/0001 awarded by the U.S. Air Force. The Government hascertain rights in the invention.

This work was also supported by the U.S. Air Force under contractF29601-98-D-0190 and by the U.S. Navy under contract N00178-04-C-3045.The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of thesecontracts.

FIELD OF THE INVENTION

The invention relates generally to high-power optical amplifiers andlasers and more particularly to methods and apparatus applicable forphotonic-crystal optical fibers and similar structures suitable for veryhigh peak-power and average-power optical output,near-diffraction-limited beam quality, multi-kHz pulse-repetition rate,highly controlled spectral properties including narrow line width andhigh signal-to-noise ratios.

BACKGROUND OF THE INVENTION

Rare-earth (RE) doped, pulsed fiber lasers and amplifiers constituteefficient and compact optical sources that can emit adiffraction-limited Gaussian beam of highly controlled spectral quality.The output power generated by these sources is limited, however, byparasitic nonlinear optical effects, amplified spontaneous emission, anddamage to optical components due to high optical power.

Nonlinear effects include stimulated Brillouin and Raman scattering (SBSand SRS), self- and cross-phase modulation (SPM and XPM), and four-wavemixing (FWM). The common origin of these effects is the high opticalintensity in the fiber core and long path for the nonlinear interactionbetween the in-fiber optical beam and fiber material (e.g., silica).These effects hamper in particular the generation of high-peak-powerpulses by causing unwanted spectral broadening, distortion of the pulsetemporal profile, and sudden power instabilities that result in opticaldamages.

The build-up of amplified spontaneous emission (ASE) is due to the highoptical gain available in the fiber core in the time interval betweenpulses. ASE constitutes an unwanted continuous-wave (CW) noise, whichdegrades the pulse/background contrast and, most importantly, limits theattainable pulse energy by using up gain.

Finally, optical damages can occur in the fiber because of materialbreakdown in the presence of high optical intensities. The fiber facetsare especially vulnerable because exposed to potential contaminants andsubject to defects that can initiate damage.

There is a need for fiber lasers and optical amplifiers configured toemit pulses of considerably higher energy and peak power than currentlyavailable. These sources must be designed so as to circumvent thelimitations described above.

SUMMARY OF THE INVENTION

In some embodiments, the present invention provides one or moreoptical-pulse amplifiers based on photonic-crystal-fiber technology,which simultaneously provide one or more of the following: pulse peakpower in excess of 1 megawatt (MW), near-diffraction-limited beamquality (M²<1.5), multi-kHz pulse-repetition rate, and highly controlledspectral properties that include, in some embodiments, pulse linewidthof 50 GHz or less and signal-to-noise ratio of 30 dB or more. M² is awidely used dimensionless beam-propagation-quality parameter and thedefinition adopted hereafter is the same provided in the current ISOStandard for beam quality characterization (ISO 11146). For a pureGaussian beam, M²=1. In this document, signal-to-noise ratio is definedas the intensity ratio between the pulse's spectral peak and that ofbackground radiation at wavelengths other than those of the pulse.

In some embodiments, the present invention provides pulsed fiber lasersand amplifiers for applications that require pulses that are a fewnanoseconds long at multi-kHz pulse-repetition rates (PRR), exhibitingone or more of the following characteristics: high peak power (usefulfor applications such as, e.g., wavelength conversion, materialsprocessing, and ranging), high pulse energy (useful for applicationssuch as, e.g., illumination and imaging), and narrow spectral linewidth(useful for applications such as, e.g., remote sensing and wavelengthconversion). The present invention provides fiber-based sources thatgenerate higher pulse energies and peak powers than are conventionallyavailable, while also achieving compactness, efficiency, and high beamquality.

In some embodiments, the present invention provides high-power pulsedfiber lasers and amplifiers based on photonic-crystal-fiber technology,which produce high peak power (>500 kilowatt (kW)) linearly polarizedoutput beams of near-diffraction-limited beam quality and narrowspectral linewidth that can be effectively used for generation ofhigh-peak-power visible and ultraviolet radiation by means of frequencyconversion in nonlinear crystals. In some embodiments, the outputs fromseveral linearly polarized, spectrally narrow, high-peak-power fiberlasers and/or amplifiers based on the same technology and arranged in asuitable pattern can be combined spectrally by using an externaldispersive optical element to produce a beam of near-diffraction-limitedbeam quality and peak power/pulse energy approximately equal to the sumof the peak powers/pulse energies from each individual fiberlaser/amplifier. The benefit of this beam-combination scheme is that itproduces a peak power in a single beam that is much higher than thedamage threshold for an individual fiber.

Optical fibers are waveguides, in which a certain number of transversemodes of radiation can exist and propagate with low loss. Differenttransverse modes correspond to different transverse profiles of opticalintensity. In step-index fibers, the fundamental mode profile is verysimilar to a Gaussian. Fibers that support only this mode (usuallyreferred to as “single-mode fibers”) inherently produce the best beamquality. For a given refractive index step between core and cladding,the number of transverse modes supported by a fiber is proportional tothe core diameter. Therefore, large-core fibers tend to be multimode,which degrades the beam quality.

Moreover, in multimode fibers, thermal and mechanical perturbations caneffect uncontrolled changes in the mode pattern and beam pointing. Inlarge-core RE-doped fibers, different modes exhibit different spatialoverlap with the dopant distribution, and hence different modesexperience different gain, therefore sudden mode pattern changes resultin power instabilities. These instabilities can also result in suddenintensity spikes that damage the fiber facet or body, especially infiber lasers/amplifiers emitting high peak power (e.g., in the range of1 MW or higher). The present invention addresses these problems, amongothers.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A is a block diagram of a master-oscillator/power-amplifier (MOPA)system 100 having high-peak-power optical amplifiers including one ormore gain stages, pump blocks and rare-earth-dopedphotonic-crystal-fiber (PCF) power amplifiers.

FIG. 1B is a schematic diagram of a system 101 having high-peak-poweroptical amplifiers including one or more gain stages, pump blocks andrare-earth-doped photonic-crystal-fiber (PCF) power amplifiers.

FIG. 1C is a block diagram of a pump block 118.

FIG. 1D is a schematic diagram of a pump block 118.

FIG. 1E is a schematic diagram of a compact system 102 havinghigh-peak-power rare-earth-doped photonic-crystal-fiber(PCF)/photonic-crystal-rod (PCR) optical power amplifiers.

FIG. 1F is a schematic diagram of a compact system 103 havinghigh-peak-power rare-earth-doped PCF/PCR optical power amplifiers.

FIG. 1G is a schematic diagram of a pump block 119.

FIG. 1H is a schematic diagram of a compact system 104 havinghigh-peak-power rare-earth-doped PCF/PCR optical power amplifiers.

FIG. 1I is a schematic diagram of a pump block 121.

FIG. 2 is a schematic diagram of a high-peak-power rare-earth-dopedphotonic-crystal-rod optical power amplifier 200.

FIG. 3A is an end-view schematic diagram of a high-peak-powerrare-earth-doped photonic-crystal rod 300 having a beam-expandingendcap.

FIG. 3B is a side-view schematic diagram of PCR 300 having abeam-expanding endcap.

FIG. 3C is a cross-section schematic diagram of PCR 300 having abeam-expanding endcap.

FIG. 3D is an end-view schematic diagram of a high-peak-powerrare-earth-doped photonic-crystal rod 301 partially fabricated intohaving a beam-expanding endcap.

FIG. 3E is a side-view schematic diagram of PCR 301.

FIG. 3F is a cross-section schematic diagram of PCR 301.

FIG. 3G is an end-view schematic diagram of a high-peak-powerrare-earth-doped photonic-crystal rod 302 partially fabricated intohaving a beam-expanding endcap after tapering.

FIG. 3H is a side-view schematic diagram of PCR 302.

FIG. 3I is a cross-section schematic diagram of PCR 302.

FIG. 3J is a microphotograph of PCR 304 after tapering the end andbefore removing the epoxy cap and collapsing the air cladding to formthe beam-expanding endcap.

FIG. 4A is a side-view schematic diagram of a high-peak-powerrare-earth-doped photonic-crystal rod (PCR) 401 partially fabricatedinto having a beam-expanding endcap.

FIG. 4B is a side-view schematic diagram of PCR 402 after partiallycollapsing the holey region of the endcap.

FIG. 4C is a diagram of PCR 403 after further collapsing the holeyregion of the endcap.

FIG. 4D shows PCR 404 after yet more collapsing the holey region of theendcap.

FIG. 4E shows PCR 404 after angle polishing the end of the endcap toform the rod facet.

FIG. 4F is a side-view schematic diagram of PCR 405 after injecting anindex matching compound into the air cladding at the rod end andpolishing the rod end to form a beam-expanding endcap and facet.

FIG. 5A is a side-view schematic diagram of a high-peak-powerrare-earth-doped photonic-crystal fiber (PCF) 500 after faceting andcollapsing the holey region of the endcap.

FIG. 5B is a microphotograph of PCF 500.

FIG. 6 is a microphotograph of a photonic-crystal rod (PCR) 600 afterfaceting and collapsing the holey region of the endcap.

FIG. 7A is a side-view block diagram of a system 700 for forming abeam-expanding endcap onto high-peak-power rare-earth-dopedphotonic-crystal rod 310 at a time before collapsing the holey region ofthe endcap.

FIG. 7B is a side-view block diagram of a system 700 at a time aftercollapsing the end portion of the holey region of the endcap.

FIG. 7C is a side-view block diagram of a system 700 at a time aftermoving the endcap further into the heating region.

FIG. 7D is a side-view block diagram of a system 700 at a time afterfurther collapsing the end portion of the holey region of the endcap.

FIG. 7E is a side-view block diagram of photonic-crystal rod 310 at atime after collapsing the holey region of the endcap and angle-polishingthe end.

FIG. 7F is an end-view block diagram of a system 700.

FIG. 7G is an end-view block diagram of a system 701 for forming abeam-expanding endcap onto ribbon-like high-peak-power rare-earth-dopedphotonic-crystal rod.

FIG. 8A is a cross-section-view schematic diagram of a ribbon-PCR system800 having a ribbon-like high-peak-power rare-earth-dopedphotonic-crystal rod (PCR ribbon).

FIG. 8B is a perspective-view schematic diagram of a ribbon-likehigh-peak-power rare-earth-doped photonic-crystal-rod spectral-beamcombiner output-stage system 808.

FIG. 8C is a plan-view schematic diagram of amaster-oscillator/power-amplifier (MOPA) high-peak-powerrare-earth-doped photonic-crystal-ribbon laser system 870 using aribbon-like high-peak-power rare-earth-doped photonic-crystal rodspectral-beam combiner output-stage system 808.

FIG. 8D is a cross-section-view schematic diagram of a preform 861configured to compensate for lateral shrinkage in later forming of aribbon-like high-peak-power rare-earth-doped photonic-crystal rodinner-cladding/core portion 862.

FIG. 8E is a cross-section-view schematic diagram of a ribbon-likehigh-peak-power rare-earth-doped photonic-crystal-rodinner-cladding/core portion 862.

FIG. 8F is a cross-section-view schematic diagram of a polarizinghigh-peak-power rare-earth-doped photonic-crystal rod 880.

FIG. 8G is a cross-section-view schematic diagram of asingle-polarization high-peak-power rare-earth-doped PCF or PCR ribbon881.

FIG. 8H is a cross-section-view schematic diagram of ribbon PCR 887having stress elements 886 to induce birefringence in cores 868.

FIG. 8-I is a cross-section-view schematic diagram of a central portionof a high-peak-power rare-earth-doped photonic-crystal rod 890.

FIG. 9A is a perspective-view schematic diagram of a high-peak-powerrare-earth-doped PCF or PCR ribbon MOPA laser system 900.

FIG. 9B is a plan-view schematic diagram of MOPA laser system 900.

FIG. 9C is an elevation-view schematic diagram of MOPA laser system 900.

FIG. 10 is a plan-view schematic diagram of MOPA laser system 1000having a segmented final gain section having fiber splices.

FIG. 11A is a perspective-view schematic diagram of a high-peak-powerrare-earth-doped laser-welded PCF or PCR MOPA laser system 1100.

FIG. 11B is an elevation-view schematic diagram of MOPA laser system1100.

FIG. 11C is an end-view schematic diagram of MOPA laser system 1100.

FIG. 11D is a perspective-view schematic diagram of anotherhigh-peak-power rare-earth-doped laser-welded PCF or PCR MOPA lasersystem 1101.

FIG. 11E is an elevation-view schematic diagram of MOPA laser system1101.

FIG. 11F is an end-view schematic diagram of MOPA laser system 1101.

FIG. 11G is a perspective-view schematic diagram of yet anotherhigh-peak-power rare-earth-doped laser-welded PCF or PCR MOPA lasersystem 1102.

FIG. 11H is an elevation-view schematic diagram of MOPA laser system1102.

FIG. 11-I is an end-view schematic diagram of MOPA laser system 1102.

FIG. 12A is a schematic diagram of a high-peak-power rare-earth-dopedPCF or PCR MOPA laser system 1200 having an improved delivery fiber1230.

FIG. 12B is a cross-section-view schematic diagram of an output end 1210of improved delivery fiber 1230.

FIG. 12C is a cross-section-view schematic diagram of an input end 1220of improved delivery fiber 1230.

FIG. 12D is a cross-section-view schematic diagram of an alternativeinput end 1222 of improved delivery fiber 1230.

DETAILED DESCRIPTION

Although the following detailed description contains many specifics forthe purpose of illustration, a person of ordinary skill in the art willappreciate that many variations and alterations to the following detailsare within the scope of the invention. Accordingly, the followingpreferred embodiments of the invention is set forth without any loss ofgenerality to, and without imposing limitations upon the claimedinvention.

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings that form a part hereof,and in which are shown by way of illustration specific embodiments inwhich the invention may be practiced. It is understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

The leading digit(s) of reference numbers appearing in the Figuresgenerally corresponds to the Figure number in which that component isfirst introduced, such that the same reference number is used throughoutto refer to an identical component that appears in multiple figures.Signals and connections may be referred to by the same reference numberor label, and the actual meaning will be clear from its use in thecontext of the description.

As used herein, an optical-waveguide device is defined as any devicethat provides a constrained guided optical path in a solid, for example,an optical fiber having one or more waveguide cores or an optical slabor monolithic substrate having a width and length each larger than thethickness, and having one or more waveguides formed therein (e.g.,laterally spaced waveguides formed by diffusion of a index-modifyingmaterial through a mask to form surface or near-surface waveguides). Anoptical fiber is defined as any device having one or more cores orinternal waveguides and a length much longer than a transverse width,for example a glass fiber drawn from a melt or preform or extruded froman extruder. A thin optical fiber is defined as a fiber that is thinenough to be readily bent to some non-infinite radius (e.g., aconventional optical fiber). A rod-like fiber (also referred tohereafter as “rod waveguide” or simply “rod”) is defined as a fiber thatis thick enough to readily hold its shape when released (e.g., a glassrod having a diameter of 1 millimeter (mm) or more). An optical ribbonis defined as a fiber having two or more signal cores laterally spacedacross a width of the fiber. An optical ribbon rod is defined as a fiberhaving two or more signal cores laterally spaced across a width of thefiber and that is thick enough to readily hold its shape when released.

Major factors limiting the pulse peak power and energy in pulsedfiber-based sources are parasitic nonlinear optical effects (NLEs) andinter-pulse amplified spontaneous emission (ASE).

A great deal of prior art and published literature has attempted toaddress the issue of parasitic NLEs in fiber pulse amplifiers. Theseparasitic nonlinear optical effects arise, at sufficiently high pulsepeak power, from the nonlinear interaction between the optical pulsesconfined in the fiber core and the silica-based material of the fiber.NLEs include stimulated Brillouin scattering and stimulated Ramanscattering (SBS and SRS), self- and cross-phase modulation (SPM andXPM), and four-wave mixing (FWM, also referred to as “cross-talk”).Different NLEs have the following in common:

-   -   a) the pulse optical power at which they set on (referred to as        “threshold power” and coinciding with the maximum pulse power        achievable) is proportional to the fiber core area and inversely        proportional to the fiber length. In other words, long fibers of        small core favor NLEs; and    -   b) they cause:        -   i. unwanted spectral broadening of the optical pulses and/or            wavelength conversion, and        -   ii. optical feedback, power instabilities and ensuing            potential damages to the optical components.

Because of the above-described dependence of the NLE threshold power onfiber length and core area, a widely adopted method for avoiding NLEshas been to resort to fibers featuring as large a core as possible.These fibers are best known in the art as large-mode-area (LMA) fibersand exhibit core areas more than an order of magnitude larger than thoseof conventional single-mode fibers used for telecommunications.

A major problem with this approach is that for cores that are largeenough, the cores guide several transverse modes, which degrade the beamquality (i.e., will result in a beam-quality factor M² much greater than1). Bend-loss-induced mode selection (see, e.g., U.S. Pat. No. 6,496,301issued to Koplow et al., which is incorporated herein by reference) incombination with mode-matched launching (see, e.g., U.S. Pat. No.5,818,630 issued to Fermann et al., which is incorporated herein byreference) has been shown to be a way for recoveringnear-diffraction-limited (M² of about 1) beam quality in multimodefibers of medium core size (20 to 30 microns in diameter). However, asthe core area and diameter increases, the bend-induced discriminationbetween fundamental and higher-order mode becomes marginal, whichresults in either poor beam quality (if there is loose coiling of thefiber) or a large efficiency penalty (if there is tight coiling of thefiber). To continue increasing the core area without beam-quality orefficiency penalties, special fiber designs are ultimately required,regardless of the methods used.

In some embodiments, the present invention uses photonic-crystal fibers(PCFs) as an example of these special designs. As is well known in theart (see, for example, J. Limpert et al., Optics Express 12, 1313-1319(2004) and G. Bonati et al., Late Breaking Developments—Session 5709-2a,Photonics West 2005 (San Jose, Calif.), each of which is incorporatedherein by reference), the internal air/silica microstructure of PCFsaffords finer control on the refractive-index profile than is possiblein standard fiber-manufacturing processes.

Some embodiments of the present invention optionally include certainaspects disclosed in U.S. Provisional Patent Application 60/703,824filed on Jul. 29, 2005, titled “PERIODIC FIBER TO SUPPRESS NONLINEAREFFECTS IN RARE-EARTH-DOPED FIBER AMPLIFIERS AND LASERS,” which ishereby incorporated by reference in its entirety.

Some embodiments of the present invention provide highly efficientrare-earth-doped single-mode PCFs of exceedingly large core diameter, inwhich the single-mode characteristic is enabled by precisely obtaining avery low NA in the core through careful design of the PCF's internalair/silica microstructure, without the need for additional conditionssuch as bending or special launching conditions.

In contrast to large-core, rare-earth-doped PCFs that have been used asamplifiers to generate an output beam of high average power (see, forexample, J. Limpert, A. Liem, M. Reich, T. Schreiber, S. Nolte, H.Zellmer, A Tünnermann, J. Broeng, A. Petersson, and C. Jakobsen,“Low-nonlinearity single-transverse-mode ytterbium-doped photoniccrystal fiber amplifier,” Optics Express 12, 1313-1319 (2004), G. Bonatiet al., “Late Breaking Developments,”—Session 5709-2a, Photonics West2005 (San Jose, Calif.), and F. Roser et al. Optics Letters 30,2754-2756 (2005), each of which is incorporated herein by reference)(for which NLEs do not pose a severe limitation), their use to generatehigh-peak-power optical pulses (power of 1 MW or greater) exhibitingminimal NLEs and beam quality M² of about 1 had never been demonstratedor patented to date and no prior art has been found on this topic.Indeed, the first report of megawatt-peak-power generation from arare-earth-doped photonic-crystal-fiber (PCF) amplifier was published bythe inventors (F. Di Teodoro and C. D. Brooks, Optics Letters 30,2694-2696 (2005)).

If single-mode PCFs of core diameter up to about 40 microns (μm) canstill be bent tight enough to suit practical packaging requirements,excessive bend loss characterizes those with larger cores.

The present invention, in some embodiments, uses a simple way tocircumvent the issue of bend loss while maintaining intrinsicsingle-mode operation in very large cores, to with, to avoid bendingaltogether by designing the PCF to have a straight core, either byholding the fiber so its core is straight, or forming the core in a rod.

The concept of a photonic-crystal rod was introduced very recently byLimpert and coworkers, who described a rod-like photonic-crystal-fiberlaser generating average output power in excess of 120 W (J. Limpert etal., Optics Express 13, 1055-1058 (2005)) as well as a Q-switched lasergenerating 0.5 mJ pulse energy (J. Limpert et al., Applied Physics B:Lasers and Optics 81, 19 (2005)). An important finding of this work wasthat, despite the short length, the rod did not require activewaste-heat removal owing to its very large glass overcladding (about 1.7mm diameter), which helped maintain a large surface/active-volume ratio.The overcladding was also instrumental in minimizing bend loss and,since the rod is not to be bent, an external polymer jacket (used formechanical support in standard fibers that are to be coiled) is notrequired, which improves power handling. (Most jacket polymers degradequickly and ignite at temperatures of greater than 100 degrees C. Thesetemperatures are not unusual at the outer surface of fibers generatinghigh average power. Temperatures are especially high in gain fibersexhibiting very efficient pump-light absorption, in which most of thepump light is absorbed (and most of the heat is generated) over a shortlength.)

To date, reported photonic-crystal rods featured core diameter of nolarger than about 35 μm (J. Limpert et al., Optics Express 13, 1055-1058(2005)) and the inventors know of no prior art on the use of such rodsfor high-peak-power pulse amplification. The inventors have demonstratedthe use of a 70-mm-core-diameter Ytterbium (Yb) doped photonic-crystalrod to generate optical pulses of peak power greater than three (3) MW,near-diffraction-limited beam quality (M² of about 1), negligible NLEs,and narrow spectral linewidth (<13 GHz). The inventors have furtherdemonstrated the use of a 100-mm-core-diameter Yb-doped photonic-crystalrod to generate optical pulses of peak power greater than four (4) MW,near-diffraction-limited beam quality (M² of about 1.3), negligibleNLEs, and narrow spectral linewidth (<20 GHz).

Rare-earth or other suitable dopants (selected for the desired signalwavelengths and/or pump wavelengths) are used in the optical amplifiersof the present invention. In some embodiments, Yb-doped fibers are usedfor signal wavelengths of about 1.0 micron, Erbium (Er)-doped orEr/Yb-codoped fibers are used for signal wavelengths of about 1.5microns, and Thulium (Tm)-doped or Tm/Holmium (Ho)-codoped fibers areused for signal wavelengths of about 2.0 microns or longer.

In some embodiments, the detrimental amplified spontaneous emission(ASE) is addressed by using gain staging with inter-stage spectralfiltering (short, large-core-diameter PCF amplifiers each separated fromthe next by narrow-linewidth optical filters). The spectral filterrejects most of the ASE generated in the previous stage, thus disruptingthe build-up of ASE from stage to stage. Moreover, the spectral filterrejects also backward-propagating ASE, thus providing optical isolationfor the prior stages and preventing detrimental effects such asparasitic lasing and depletion of stored energy that is meant for theforward-traveling pulses. In some embodiments, each PCF amplifier isshort to prevent buildup of non-linear effects, and the pump-claddingdiameter is kept small (presenting a low ratio of pump-cladding diameterto core diameter, for example, a ration of 3 or less) in order toincrease the rate of pump-light transfer from the pump cladding into thecore. In addition, each amplifier stage is operated in saturated orclose-to-saturated regime, corresponding to a regime in which the seed(i.e., the input) power injected in each amplifier stage is high and thegain provided by the amplifier stage is relatively low (for example <15dB) compared to that (>30 dB) usually realized, for example, inlow-output-power fiber amplifiers for telecommunications. This regime ofoperation minimizes the build-up of ASE generated within each stage.Overall, the gain-staged architecture described here is instrumental formaximizing energy storage and extraction, thus attaining high pulseenergy and high peak power.

As used herein, the term “optical amplifier” includes both devices thatamplify an optical signal from an external source (such as amaster-oscillator seed laser), as well as gain media that form internalportions of lasers and thus amplify one or more self-generated lasermodes.

FIG. 1A is a block diagram of a MOPA system 100 having high-peak-poweroptical amplifiers including one or more gain stages, pump blocks andrare-earth-doped photonic-crystal-fiber (PCF) power amplifiers. In someembodiments, system 100 includes a narrow-band master oscillator (MO)110, that, in some embodiments, optionally includes a narrow-bandband-pass filter (BPF, such as shown in FIG. 1B) tuned to the wavelengthof interest for the laser signal, and/or a one-way optical isolator 112.The optical output of MO 110 is amplified by preamplifier 115A, to whichpump laser 116A provides optical pump power. The optical output ofpreamplifier 115A is filtered by a narrow-band band-pass filter (BPF)114B and input to amplifier 115B.

In some embodiments, each amplifier stage is separated from the next bya “pump block,” hereafter defined as a monolithic interconnecting unitthat comprises at least three fiber inlets (one for the output end of anamplifier stage, one for the input end of the next amplifier stage, andone for the output end of the pump-laser delivery fiber), awavelength-discriminating optical component (for example, a dichroicfilter or band-pass filter) that reflects the pump-laser wavelength andtransmits the pulse wavelength (or vice versa), and a narrow-bandband-pass filter that transmits the pulse wavelength and rejects ASE.Here, the definition “monolithic” is intended as “made of componentsthat are optimally aligned upon assembly and do not require or allowre-alignment or further mechanical adjustment.” In some embodiments, twoor more of the components of the pump block are laser-welded to asuitable substrate. In some embodiments, the substrate and otherpump-block components are enclosed in a sealed housing 109 (see FIG.1C). The fiber inlets include, in turn, a fixture, ferrule, or otherprovision to hold the fiber in place and a lens placed at a distancefrom the fiber facet approximately equal to the lens focal length. Thetwo lenses facing the output end of an amplifier stage and input end ofthe next amplifier stage, respectively, constitute an imaging opticalsystem, which images the output beam from an amplifier stage into theinput facet of the fiber constituting the next amplifier stage. Thisoperation is also referred to as “coupling” or “injecting” light intothe next fiber amplifier. In some embodiments, for effective coupling,the direction of propagation of the beam coupled into the fiberamplifier must lie along the axis of the acceptance cone of the fiber'score, the half-divergence angle of the beam must not exceed thenumerical aperture of the fiber's core, and the diameter of the imagebeam must not exceed the fiber's core diameter. In some embodiments,these lenses exhibit different focal lengths to provide magnification ofthe beam prior to injection into the next fiber amplifier. The twolenses facing the end of an amplifier stage and output end of thepump-laser delivery fiber, respectively, form an imaging optical system,which images the pump-laser beam into the pump cladding of the fiberused in the amplifier, which is referred to as “cladding-pumping theamplifier.” In some embodiments, the pump-laser beam is coupled into theoutput end of a fiber amplifier (“backward pumping”). In someembodiments, the pump beam is imaged onto the core of the fiber in theamplifier (“core pumping”). For core-pumping, the prescriptions foreffective coupling given above apply as well. For cladding pumping, theprescriptions for effective coupling given above apply as well, providedthat the word “core” is replaced by “pump cladding”. In someembodiments, the lenses in the pair exhibit different focal lengths tode-magnify the pump-laser beam.

In some embodiments, an actual optical isolator (e.g., a device thatincludes a polarizer, Faraday rotator, and a cross-polarizer) isinstalled in series with one or more of the band-pass filters to provideisolation also against backward-traveling light of wavelengths withinthe pass band of the filters. In some embodiments, the pump blockcontains a half-wave plate that appropriately rotates the polarizationof the pulses in such a way that the polarization is parallel to theslow optical axis (i.e., the axis of higher refractive index) of thefiber used in the successive amplifier stage, which preservespolarization from stage to stage. In some embodiments, the pump blockcontains a polarizer. In some embodiments, the pump block contains abeam splitter highly transmissive at the pulse wavelength, which is usedto deflect a small amount of the pulse power towards a diagnosticelement (e.g., photo-detector), which is used to monitor the systemperformance.

In some embodiments, one or more optical components forming the pumpblock are butt-joined and welded to an enclosure by localized heatingwith a CO₂ or Nd:YAG laser, according to welding methods well known tothose skilled in the art and described in several patents (including,for example, Barnes et al., U.S. Pat. No. 4,424,435). In someembodiments, the whole enclosure or at least the portion of theenclosure onto which the component is welded consists of silica glass orother material having minimum coefficient-of-thermal-expansion (CTE)mismatch with respect to the material of the optical component itself,which avoids build up of thermally induced mechanical stress and cracks.In some embodiments, a non-optical surface of one or more components ismetallized (i.e., coated with a metal such as gold or copper) andsoldered to metallic parts of the enclosure with methods known to thoseskilled in the art. In some embodiments, one or more components are heldin place by mechanical restraints attached to the enclosure. In someembodiments, the fiber inlets consist of fixtures each including a keyedreceptacle for a fiber connector and a lens holder. In some embodiments,one or more fibers connected to the pump block are terminated byepoxy-free connectors featuring a glass ferrule laser-welded to thefiber tip. In some embodiments, the ferrule is made of glass of lowersoftening point (i.e., lower temperature at which the glass softens)compared to the material used in the fiber. In these embodiments, thefiber tip is bonded to such ferrule by inserting the fiber into theferrule and heating (using, for example, a torch or electric discharge)the ferrule to reach its softening point, which results in shrinkage ofthe inner channel of the ferrule (which hosts the fiber) under surfacetension and ensuing adhesion to the fiber. After sufficient intermediaryamplification, the output signal beam 199 is formed by BPF 114C, outputamplifier 115C, and pump 116C.

FIG. 1B is a schematic diagram of a system 101 (corresponding to oneembodiment of system 100 of FIG. 1A) having serially coupledhigh-peak-power optical amplifiers including one or more gain stages,pump blocks and rare-earth-doped photonic-crystal-fiber (PCF) poweramplifiers. In some embodiments, system 101 includes a master oscillator(MO) 110 that, in some embodiments, optionally includes a narrow-bandband-pass filter (BPF) tuned to the wavelength of interest for the lasersignal, and/or a one-way optical isolator 112. The optical output of MO110 is amplified by preamplifier 115A, which has a pump block 118B atits signal output, to which pump laser 116A provides optical pump powervia dichroic mirror/beamsplitter 113A (which highly reflects light ofthe pump wavelength and highly transmits light of the signal wavelength)and whose output is filtered by BPF 114B. In some embodiments,preamplifier 115A includes a photonic-crystal fiber having a core and/orinner cladding whose lateral extent is defined by holes extendingaxially along the fiber length surrounding the core, and having a lownumerical aperture (NA), in order that the fiber is able to be operatedin a single transverse mode. The optical output of preamplifier 115Athrough pump block 118B is input to amplifier 115B. After sufficientintermediary amplification, the output signal beam 199 is formed byoutput BPF 114C, amplifier 115C, and pump laser 116C.

In some embodiments, the master oscillator 110 produces single-frequency(i.e., single longitudinal mode), time-gated, near-transform-limited (atransform-limited pulse exhibits the narrowest possible spectrallinewidth consistent with the pulse duration), few-ns-duration (e.g.,5-ns or less, in some embodiments) pulses. In some embodiments, theshort pulse duration avoids SBS. The threshold power for SBS is thelowest (by about two orders of magnitude) among optical non-lineareffects in silica fibers. For reasons that are well explained in severalreferences (see G. P. Agrawal, Nonlinear Fiber Optics, Third Edition(Academic, San Diego, Calif., 2001) and F. Di Teodoro et al., OpticsLetters 27, 518-520 (2002)), pulse durations of a few nanoseconds orshorter eliminate the SBS problem and are therefore important or evencrucial for some embodiments of the present invention's high-peak-powerpulse amplifiers. In some embodiments, the time-gated nature of themaster oscillator—resulting in high pulse-versus-background contrast(i.e., high power extinction between pulses and CW background emissionin the interval between pulses)—avoids seeding, within the PCF, ofamplified spontaneous emission (ASE) and helps minimize the build up ofASE throughout the fiber-amplifier chain. In some embodiments, thesingle-frequency nature of the master-oscillator pulses preventsspectral broadening of the pulses as they are amplified through theamplifier chain, by avoiding cross-talk effects that may occur in thecase of closely spaced (e.g., less than 10 GHz) longitudinal modesobservable in the master oscillator's output spectrum. A major physicalmechanism behind cross talk is four-wave mixing (FWM) among thelongitudinal modes. FWM results in the redistribution of the pulseenergy over new wavelength components, which broadens the spectrum. Witha multi-longitudinal-mode master oscillator, significant FWM can occureven in the first amplifier stage of a multi-stage chain of amplifiers,and more so in successive stages. In some embodiments, thesingle-frequency nature of the master-oscillator pulses preventsspectral broadening of the pulses as they are amplified to high peakpower by avoiding cross-phase modulation (XPM). In some embodiments, thesingle-frequency nature of the master-oscillator pulses prevents therun-away spectral broadening that would result from FWM involvingmultiple longitudinal modes generated by a multi-longitudinal-mode(i.e., not single-frequency) master oscillator, the spacing betweenmodes being sufficiently small for FWM to occur efficiently when themaster oscillator pulses are amplified to high peak power.

In some embodiments, the master oscillator 110 is a single-frequency,Q-switched solid-state microchip laser (e.g.,Neodymium-Lanthanum-Scandium-Borate (Nd:LSB) microchip laser Model#STA-01-5 available from STANDA, P.O. BOX 377, 03012 Vilnius, Lithuania)as reported by the inventors in C. D. Brooks et al., Optics Express 13,8999-9002 (2005). In other embodiments, an Nd:YAG microchip laser, suchas a JDS Uniphase “Nanolase,” may be used. In yet other embodiments, anyother suitable seed laser may be used. In some embodiments, the masteroscillator 110 is a solid-state laser featuring a very short resonantcavity (e.g., Neodymium-Yttrium-Aluminum garnet (Nd:YAG) microchip laserModel # NP-15010-400 available from, e.g., Teem Photonics, 888 WorcesterStreet, Suite 260, Wellesley Mass. 02482, U.S.A.), such that even if thecavity supports multiple longitudinal modes (i.e., the laser is notsingle-frequency) the spectral separation between longitudinal modes islarge enough (about 50 GHz or more) that effective cross-talk (i.e.,FWM) does not occur in the fiber, due to phase mismatch between themodes, as reported by the inventors in F. Di Teodoro et al., OpticsLetters 30, 2694-2696 (2005) and F. Di Teodoro et al., Optics Letters30, 3299-3301 (2005).

In some embodiments, some other types of Q-switched diode-pumped bulksolid-state laser or Q-switched fiber laser can be used. In someembodiments, the master oscillator is a CW laser (e.g., in someembodiments, the 1.06-μm-wavelength diode laser model LC96A1060manufactured by Bookham (Bookham Worldwide Headquarters, 2584 JunctionAve., San Jose, Calif. 95134, www.bookham.com), or, in still otherembodiments, the fiber laser model “Boostik” Y10 manufactured by Koheras(KOHERAS A/S, Blokken 84, DK-3460 Birkerød—Denmark, www.koheras.com)),which is externally modulated by using an external electro-optic oracousto-optic modulator used as an optical “chopper” to provide therequisite optical pulses. In some embodiments, time gates (e.g.,acousto-optic or electro-optic modulators) synchronized with the masteroscillator pulses are inserted between the amplifier stages to increasesuppression of the CW background. In some embodiments, the masteroscillator is a gain-switched laser (e.g., a CW diode laser driven by atemporally modulated electric current). In some embodiments the masteroscillator is replaced by a spectrally broadband optical source (e.g.,fiber ASE source similar to the “Scorpion” model manufactured by NPPhotonics), the output of which is transmitted through an opticalband-pass filter and a modulator to produce pulses of select spectrallinewidth and temporal characteristics.

In some embodiments, the first preamplifier stage includes an amplifyingfiber from Liekki having a core diameter of 32 microns (μm), a claddingdiameter of 250 microns and a 2-meter length arranged in a10-cm-diameter coil. In some embodiments, an inter-stage bandpass filtermanufactured by Barr Associates is used for ASE rejection. In someembodiments, this configuration is then coupled to an output PCF stagehaving a PCF amplifying fiber having a core diameter of 40 microns (μm),a cladding diameter of 170 microns and a 1.5-meter length. In someembodiments, the output dichroic mirror/beamsplitter 113C is a long-passfilter that is highly reflective at 976 nm and highly transmissive at1064 nm, and the path from pump 116C includes a short-wavelength-passfilter that is highly transmissive at 976 nm and highly reflective at1064 nm.

In some embodiments, the preamplifier stages are omitted, and amicrochip laser (such as, for example, a JDS Uniphase “Nanolase” type)is directly coupled to the output PCF stage having a polarizing PCFamplifying fiber having a core diameter of 41 microns (μm), a claddingdiameter of 200 microns and a 3-meter length. In some embodiments, thepolarizing PCF amplifying fiber includes a plurality of stress-inducingrods in the hole array (e.g., at two opposite sides of the core) thatpreferentially promote one polarization state (see, e.g., FIG. 8Fbelow).

In some embodiments, the final amplifier 115C (and, in some embodiments,other earlier amplifiers as well) is formed as a substantially straightphotonic-crystal rod (PCR), with a very low core NA, which would haverelatively high bending losses if it were not operated and maintainedsubstantially straight. In some embodiments, the ends of thephotonic-crystal-rod holes that define the core and/or inner claddingare collapsed at the output end of the PCR (note the small gap 125between the right-hand end of the schematically illustratedphotonic-crystal-rod holes and the right-hand end of the rod outline),in order that the output laser beam will start to diverge before exitingthe PCR, thus reducing the power density of the output laser beam as itencounters the exit facet of the PCR 115C, and reducing the probabilityof damaging the facet due to the laser-beam power. In some embodiments,the PCR used as the final or earlier amplifier stage is segmented intotwo or more successive pieces that are laid out at an angle with respectto each other so as to provide a more compact form factor. In someembodiments, each pair of PCR segments are joined by using a short piece(e.g., about 1-cm length, in some embodiments) of bridge fiber (see,e.g., bridge fiber 1020 of FIG. 10) characterized by (a) core diameterequal to or larger than the core diameter in the PCR, (b) pump-claddingdiameter and numerical aperture equal to or higher than those of the PCRpump cladding, (in contrast to other embodiments of the presentinvention wherein the pump-cladding diameter is smaller than the PCRpump cladding as is the case for bridge fiber 1020 of FIG. 10) and (c)core NA higher than the PCR core NA such that the piece of bridge fibercan be bent at a desired radius with negligible bend loss. In someembodiments, the bridge fiber is bent to a 180-degree angle in order toplace the successive PCR segments parallel to one another (side-by-sidesuch that their cores are non-co-linear) and achieve a small physicalfootprint.

In some embodiments, the piece of bridge fiber is fusion-spliced to bothPCR segments and guides light from one PCR segment to the next with lowloss. Although the core NA of the bridge fiber is much higher than thatof the PCR, which enables multiple transverse modes to be guided in thebridge-fiber segment, the fundamental mode in the PCR exhibits highspatial overlap with transverse modes in the bridge fiber that exhibit asingle central maximum, such as the fundamental mode. These modes aretherefore preferentially excited. As these modes approximate a Gaussianprofile, their M² value is close to 1, which leads to minimumdegradation in the optical brightness of the PCR beam as it enters thebridge fiber. Since negligible mode scrambling occurs given the veryshort length of the piece of bridge fiber, the optical brightness of thebeam exiting the PCR is preserved to a large extent as it travelsthrough the whole length of the bridge fiber. As a result, at the splicemarking the transition from the bridge fiber into the next segment ofPCR, the splice loss is low, which allows for high-efficiency operationof the segmented amplifier. In other embodiments (rather than using abridge fiber as just described), light from one PCR segment to the nextis transmitted using optical micro-components (e.g., a collimating lens,a mirror, and a focusing lens). In some embodiments, since each segmentprovides only a fraction of the optical gain of the PCR as a whole, BPFsare not required between segments of a single PCR amplifier stage, asthey are used, instead, between amplifier stages.

In some embodiments, the present invention includes one or morephotonic-crystal-fiber (PCF) amplifiers having a very large-mode-area(LMA) core while maintaining a single transverse mode for the signalbeam in the core. In some embodiments, a master oscillator is used toobtain a temporal sequence of narrow-linewidth short-duration opticalpulses that are amplified through a series of PCF power-amplifierstages, each separated from the prior stage by a one-way opticalisolator and/or a narrow-linewidth band-pass filter (e.g., pump block118B).

In some embodiments, the optical-source design based on themaster-oscillator/power-amplifier (MOPA) architecture (FIGS. 1A, 1B)permits production of high-power pulses (in excess of 1-MW peak power),while retaining diffraction-limited beam quality, spectrally narrowoutput, linear polarization, and high efficiency. In some embodiments,target applications include lidar (i.e., light detection and ranging),illumination, remote sensing, high-speed marking, harmonic generation(e.g., visible and/or UV light), and pumping of optical parametricoscillators (OPOs), just to name a few.

A transform-limited pulse (also known as a Fourier-transform-limitedpulse or a bandwidth-limited pulse) is a pulse of a wave that has theminimum possible duration for a given spectral bandwidth, since anyshorter pulse will have a broader spectral bandwidth due to Fouriertransformation of the pulse shape onto other spectral frequencies. Insome embodiments, transform-limited pulses have a constant phase acrossall frequencies making up the pulse. Any waveform can be disassembledinto its spectral components by Fourier transformation. The length of apulse is determined by its complex spectral components, which includenot just their relative intensities, but also the relative positions(spectral phase) of these spectral components.

In some embodiments, the invention uses master oscillators featuringvery short cavities (e.g., microchip lasers), such that the inter-modalspectral separation is large enough (about 50 GHz or more) thateffective four-wave mixing (FWM) does not occur in the fiber, due tophase mismatches between the modes. However, even with this type ofmaster oscillator, some embodiments avoid multi-longitudinal modes inorder to reduce or suppress at least two undesirable results: First, FWMcan become phase-matched in the case where the light propagating in thefiber is in a well-defined polarization state. Second, uponamplification to high peak powers, the presence of multiple modes leadsto nonlinear cross-phase modulation, which, in turn, causes spectralbroadening. Further, even in single-frequency master oscillators, phaseor amplitude fluctuations (which cause departure from transform-limitedbehavior) strongly favor nonlinear self-phase modulation, which, again,broadens the spectrum.

Because of the very high gain achieved in some embodiments of fiberamplifiers, build-up of inter-pulse ASE is possible in CW-pumped fiberamplifiers seeded by pulses of repetition rates in the kHz range. Inideal conditions, ASE would be seeded only by photons spontaneouslyemitted by the rare-earth dopants along the fiber and captured in thecore. Typically, this yields a very small amount of ASE optical power.However, if the pulsed master oscillator 110 produces some appreciableamount of undesirable CW background light (i.e., between the pulses thatare desired), then this CW light will also be amplified in the fiber,resulting in much higher ASE power at the cost of limiting the energyavailable to amplify the pulse. This CW background from the masteroscillator is especially detrimental because it travels through theentire fiber length, and thus experiences higher gain than spontaneousemission from within the amplifying fibers (only spontaneous-emissionphotons generated near the fiber's entry facet experience the wholefiber length's gain). Moreover, since at least a fraction of the CWbackground light has the same wavelength as the pulse (i.e., the CWlight is “in-band”), it cannot be spectrally removed by filtering, evenin multi-stage amplifiers. A background-free pulsed master oscillator istherefore very desirable in some embodiments of the present inventionfor pulse amplifiers that are subject to ASE build-up. In someembodiments, a Q-switched laser is used as an optical source that haslittle or no CW background signal between pulses. In fact, typicalQ-switched lasers are, by their nature, background free—a pulse isgenerated only when an intracavity switch is turned to the “open”position. Typically, when the switch is closed, negligible light leaksout of the cavity.) In some embodiments, the power amplifier (e.g., thatportion of system 101 to the right of master oscillator 110) isconfigured as a multi-stage chain of rare-earth-doped fibers, each stagebeing end-pumped in a backward direction by a fiber-coupled diode-pumplaser. With pump light counter-propagating with respect to the amplifiedpulses, two benefits, both either important or even crucial forhigh-power pulse amplification, are obtained: First, the pulse-energygrowth along the fiber is nearly exponential and therefore the amplifiedpulse attains its maximum peak power very close to the fiber outputfacet, which minimizes the silica-to-high-power-pulse interaction lengthfor nonlinear generation. Second, most of the pump power, and thus gain,is made available in the final portion of the fiber encountered by thesignal and is more effectively extracted by the signal pulse (alreadyamplified in the first portion of the fiber), which improves efficiency.Conversely, in co-propagating pumping, most of the gain is available atthe fiber signal-input end where the pulse exhibits the lowest power,which tends to increase the silica-pulse interaction length, thusfavoring nonlinearities.

FIG. 1C is a block diagram of an inter-stage pumping/filtering/isolatingunit 118 (also denoted as “pump block” 118 in this document). In someembodiments, at the left is the prior stage amplifier “a” that connectsusing a ferrule 117 into pump block 118 to deliver its output signal andto obtain pump light. In some embodiments, pump block 118 resides inhousing 109, which, in some embodiments, forms a monolithic opticalbench for the components therein.

In other embodiments, the various components of the pump block 118 arelaser-welded to a suitable monolithic substrate (such as a glasssubstrate that is compatible with the glass of the dichroicmirror/beamsplitter “b” and band-pass filter (BPF) “c” and any lenses(such as 112 shown in FIG. 1D)) for physical stability. In some suchembodiments, the laser-welded assembly is placed and/or sealed in ahousing 109.

In some embodiments, at the bottom is the fiber “e” that connects usinga ferrule 117 into pump block 118 to deliver pump light. Dichroicmirror/beamsplitter “b” (corresponding to dichroic mirror/beamsplitter113 of FIG. 1D) is highly transmissive for light having the signalwavelength (e.g., 1064 nm, in some embodiments) towards following stagesto the right and highly reflective for light having the pump wavelength(e.g., about 976 nm, in some embodiments) towards the prior stage to theleft. The signal light passes through spectral band-pass filter (BPF)“c” and exits pump block 118 into following amplifier stage “d” througha ferrule 117. In some embodiments, each ferrule 117 includes a two-partdisconnectable ferrule pair plug “f” and socket “g” or other suitableconnector.

FIG. 1D is a schematic diagram of a pump block 118, showing additionaldetails. In some embodiments, each optical port has one or more focusingelements 112, such as lenses “h,” to focus and shape the optical beamsfrom the respective fiber endcaps 111. In some embodiments, the signallight (e.g., 1064 nm) from endcap 111 of PCF “a” passes left to rightthrough an entry lens 112, dichroic mirror/beamsplitter 113 (“b”), BPF114 (“c”), and an exit lens 112 that focuses it into endcap 111 of PCF“d” for further amplification, while the pump light (e.g., 976 nm)passes up from the endcap 111 of pump-delivery fiber “e” through a pumplens 112 and reflects to the left off dichroic mirror/beamsplitter 113and through an entry lens 112 back into the endcap 111 of PCF “a” whereit is used to pump that previous stage.

End pumping of fiber amplifiers is a very-well-established method forpumping. However, alternatives to end pumping have been introduced inrecent years, for one or more of the following reasons:

-   -   a. Eliminating free-space optical paths (for        robustness/stability);    -   b. Free up the fiber-amplifier ends (e.g., to enable splicing);    -   c. Distributing entry points for pump light over the fiber        length (for heat-dissipation purposes); and    -   d. Increasing the pumping efficiency.        Such alternatives include V-groove side pumping and pumping        through fused fiber bundles. While ultimately relying on end        pumping, the pump-block design illustrated in FIG. 1C and FIG.        1D is an ideal solution for some embodiments of high-power pulse        amplifiers. In fact, the motivations for adopting alternative        pumping methods (“a.” through “d.”, as listed above) are either        irrelevant for this specific application or fully addressed by        pump blocks.

First, the existence of free-space optical paths in the injection ofpump light into a fiber is not undesirable, per se, but only if thecoupling of light in and out of fibers relies on generic bulk opticalmounts of limited mechanical stability and/or cumbersome footprint, andif a fiber-end treatment of sufficient quality is not available. Boththese potential issues are eliminated in the setup illustrated in FIG.1C and FIG. 1D as discussed above.

Further, while the benefits of distributing pump-injection points alongthe fiber are significant for diluting the thermal load in long fibersources (several meters or more), such as those used in high-power CWfiber lasers, they are substantially non-existent in high-power pulsedfiber sources, in which the fibers must be as short as possible tosuppress optical nonlinearities. Thermal management in such short fibersis addressed, instead, by appropriate fiber designs such as thephotonic-crystal rod described in this document.

Still further, the inventors have demonstrated that pump-couplingefficiency close to 90% (i.e., similar to the best values obtained usingfused-pump couplers) can be obtained using end pumping only.

In some embodiments, the fiber amplifier is staged (i.e., separated intoa plurality of amplification stages, each connected through a highlywavelength-selective filter to minimize ASE by having lower gain in eachfiber portion and through inter-stage spectral filtering). In someembodiments, the spectral filter acts also as an optical isolator infew-ns-duration pulse applications.

In some embodiments, each fiber-amplifier stage is backward-pumped usinga monolithic “pump block” (which does not rely on fused couplers). Insome embodiments, fibers are all connected in an epoxy-free fashion, formaximum reliability in high-power applications. In some embodiments, oneor more of the final power amplifier(s) is a rod-like photonic-crystalfiber having a core diameter of about 35 microns or more. In otherembodiments, the rod-like photonic-crystal fiber has a core diameter ofabout 40 microns or more, about 45 microns or more, about 50 microns ormore, about 55 microns or more, about 60 microns or more, about 65microns or more, about 75 microns or more, about 80 microns or more,about 85 microns or more, about 90 microns or more, about 95 microns ormore, about 100 microns or more, about 125 microns or more, about 150microns or more, about 175 microns or more, about 200 microns or more,or about 250 microns or more, just to name a few. In some embodiments,this fiber presents enabling characteristics for high-peak-powergeneration that include: a very large core having extra-low NA (e.g.,for suppression of nonlinearities and single-mode beam quality),negligible bend losses (e.g., a rod-like photonic-crystal fiber that iskept substantially straight), short length, and excellent thermalproperties.

High-Peak-Power, Linearly-Polarized, Diffraction-Limited Pulses from aLarge-Core Yb-Doped Photonic-Crystal Fiber (PCF) and Photonic-CrystalRod (PCR) and their Use to Generate High-Peak-Power UV, Visible, andInfrared Light

In some embodiments, a single-polarization, single-transverse-mode,Yb-doped PCF (i.e., a PCF that guides light only in one transverse modeand one polarization state) having large core (≧40 μm diameter) is usedin a high-peak-power pulse amplifier.

In some embodiments, the single-polarization nature of the PCF isimparted by axial elements (referred to as “stress elements”) positionedin close proximity to the core and made of material that exhibitsthermal expansion coefficient markedly different from that of the core,such that stress is accumulated in the core as the PCF is drawn and suchstress induce birefringence as described in T. Schreiber et al., OpticsExpress 13, 7621-7630 (2005) and by the inventors in F. Di Teodoro etal., Advanced Solid-State Photonics (ASSP, 29 Jan.-1 Feb. 2006, InclineVillage, Nev.) Technical Digest, Paper ME3. In some other embodiments,the single-polarization nature of the PCF stems from incorporatingfeatures that cause the PCF cross section to depart from circularsymmetry, thus inducing form birefringence.

In some embodiments, a single-polarization PCF is implemented in amaster-oscillator/power-amplifier (MOPA) optical source. In at least oneembodiment, the MOPA source consists of a master oscillator (Nd:LSBmicrochip laser Model #STA-01-5 available from STANDA, referred toabove) seeding a backward-pumped two-stage fiber amplifier, in which thePCF has a core diameter of approximately 40 microns and is featured inthe final amplifier. From this MOPA, high-energy (>0.6 mJ),high-peak-power (>600 kW), high-average-power (>6 W),1.06-μm-wavelength, 1-ns pulses in a stably linearly polarized (100:1polarized, corresponding to a degree of polarization of 99%) output beamof single-mode, near-diffraction-limited spatial quality (M²<1.3) andnarrow spectral linewidth (10 GHz) can be obtained, as reported by theinventors in F. Di Teodoro et al., Advanced Solid-State Photonics (ASSP,29 Jan.-1 Feb. 2006, Incline Village, Nev.) Technical Digest, Paper ME3.

In at least one embodiment, a single-transverse-mode, large-core,Yb-doped PCF amplifier generates 1.06-μm-wavelength,diffraction-limited, sub-ns optical pulses when seeded by a masteroscillator (Nd:YAG microchip laser Model #NP-15010-400 available fromTeem Photonics). By building this amplifier, the inventors have obtaineda peak power of greater than 700 kW (the highest in a linearly polarizedoutput from a fiber), and exhibiting a stable degree of polarization(100:1 polarized).

In some embodiments the single-polarization PCF is replaced by apolarization-maintaining PCF, i.e., a PCF that maintains thepolarization of the light launched in its input end throughbirefringence induced in the core by the same design features (stresselements or asymmetry) used in the single-polarization PCF.

In some embodiments, the MOPA source includes multiple stages ofamplification all featuring single-polarization orpolarization-maintaining fibers, with the final amplifier featuring avery-large-core (>50 mm diameter) Yb-doped photonic-crystal rod (PCR)that emits a single-transverse-mode, near-diffraction-limited outputbeam (M²<1.5) and features, in some embodiments, stress elements inclose proximity to the core, similar to those present in the PCFdescribed above or, in other embodiments, design features that departfrom cross-sectional circular symmetry (resulting in formbirefringence). By virtue of such characteristics, which make the PCRbehave as a polarization-maintaining or single-polarization waveguide,the PCR emits a linearly-polarized beam (e.g., 50:1 polarized in someembodiments, or between 50:1 and 100:1 polarized in other embodiments,or better than 100:1 polarized in other embodiments). In someembodiments, the PCR featuring stress elements exhibits core diameterbetween 50 and 100 μm. In some other embodiments, the PCR featuringstress elements exhibits core diameter >100 μm.

In some embodiments, the PCR emits a linearly polarized output beam ofwavelength in the 1.0-1,1-μm range and single-transverse-mode,near-diffraction-limited quality, and pulse peak power in excess of 1MW, pulse energy in excess of 1 mJ, and pulse spectral linewidth <50GHz.

In some embodiments, an amplifier featuring a single-transverse-mode,single-polarization or polarization maintaining, large-core (≧40-μmdiameter), Yb-doped PCF is used to generate high-peak-power (>100-kW)pulses in the visible wavelength range 500-550 nm, through frequencydoubling of the fundamental wavelength (in the range 1.0-1.1 μm) in anonlinear crystal. In some embodiments, said nonlinear crystal is apiece of lithium triborate (LBO), as reported by the inventors in F. DiTeodoro et al., Advanced Solid-State Photonics (ASSP, 29 Jan.-1 Feb.2006, Incline Village, Nev.) Technical Digest, Paper ME3 or crystal,just to name a few). In other embodiments, the nonlinear crystal couldbe a piece of potassium dihydrogen phosphate (KDP) or potassium titaniumoxide phosphate (KTP), just to name a few.

In some embodiments, an amplifier featuring a single-transverse-mode,single-polarization or polarization-maintaining, very-large-core (>50-μmdiameter), Yb-doped PCR is used to generate high-peak-power (>100-kW)pulses in the visible wavelength range 500-550 nm, by means of the samefrequency-doubling methods described in the previous paragraph.

In at least one embodiment, as reported by the inventors in F. DiTeodoro et al., Advanced Solid-State Photonics (ASSP, 29 Jan.-1 Feb.2006, Incline Village, Nev.) Technical Digest, Paper ME3, frequencydoubling in a LBO crystal of the output from an optical source featuringa 40-μm-core-diameter, single-polarization, single-transverse-modeYb-doped PCF as the final amplifier produced peak power in excess of 400kW at 531-nm wavelength in ˜1-ns pulses at repetition rate ˜10 kHz.

In some embodiments, high-peak-power visible sources obtained byfrequency doubling of high-peak-power sources featuring asingle-polarization or polarization-maintaining, large-core,single-transverse-mode, Yb-doped PCF or PCR are used formaterials-processing applications including laser marking or cutting ordrilling or conditioning of metals, semiconductors, plastics, andceramics. In other embodiments, these sources are used for medicalapplications including ablation, scarification, abrasion, or otherconditioning of live tissues.

In some embodiments, a single-transverse-mode, large-core (40-μmdiameter), Yb-doped PCF is used to generate high-peak-power (>100-kW)pulses in the UV through frequency tripling, quadrupling, andquintupling of the fundamental wavelength (in the range 1.0-1.1 μm) in anonlinear crystals (e.g., in some embodiments, two LBO crystals or oneLBO crystal and one cesium LBO (CLBO) in series, just to name a few).

In at least one embodiment, as reported by the inventors in F. DiTeodoro et al., Advanced Solid-State Photonics (ASSP, 29 Jan.-1 Feb.2006, Incline Village, Nev.) Technical Digest, Paper ME3, frequencytripling and quadrupling of the output from an optical source featuringa 40-μm-core-diameter, single-polarization, single-transverse-modeYb-doped PCF as the final amplifier produced peak power in excess of 160kW at 354-nm and 190 kW at 265.5-nm wavelength, respectively.

In some embodiments, the wavelength of the UV light generated byfrequency tripling can be in the range 333-367 nm. In some embodiments,the wavelength of the UV light generated by frequency quadrupling can bein the range 250-275 nm. In some embodiments, the wavelength of the UVlight generated by frequency quintupling is in the range 200-220 nm.

In some embodiments, an optical UV source features a very-large-core PCRsuch as the as the final amplifier before a wavelength-conversiondevice.

In some embodiments, the optical source obtained by frequency tripling,quadrupling, or quintupling of the output of a high-peak-power source offundamental wavelength in the range 1.0-1.1 μm, based on asingle-polarization or polarization-maintaining, single-transverse-mode,large-core or very-large-core Yb-doped PCF or PCR constitutes anefficient and compact high-peak-power optical source generating UV lightat power levels suitable for remote sensing of biological and chemicalagents and pollutants. In some embodiments, the biological agentsdetected contain the amino acids tryptophan, phenylalanine, andtyrosine, just to name a few, which are identified by detecting theirfluorescence emitted upon irradiation by UV light in the 250-275-nmwavelength range (Committee on Materials and Manufacturing Processes forAdvanced Sensors, National Research Council, Sensor Systems forBiological Agent Attacks: Protecting Buildings and Military Bases,National Academy Press, Washington D.C., 2005. See p. 73). In someembodiments, the chemical agents detected contain toxic compounds suchas acrolein, which is identified by detecting their fluorescence emittedupon irradiation by UV light in the 333-367-nm wavelength range. In someembodiments, this UV source represents an enabling technology forlong-range detection of airborne toxic biological and chemical agentsexecuted via the method of UV laser-induced-fluorescence (LIF) LIDAR byoptical systems incorporating the described UV source and installed onunmanned airborne vehicles (UAVs). In some embodiments, this UV sourceconstitutes an enabling technology for portable, in-the-field devices,which can be deployed in a variety of scientific, industrial, andnational-security applications.

In some embodiments, the use of a single-polarization orpolarization-maintaining, single-transverse-mode, large-core (>40-μmdiameter) Yb-doped PCF enables the efficient production ofhigh-peak-power UV light through frequency tripling, quadrupling, andquintupling. In some embodiments, the use of said PCF is instrumental inovercoming the limitations of small-core (<25-μm diameter)polarization-maintaining fibers, which incur detrimental opticalnonlinearities when used to produce high-peak-power (>300-kW) pulses andare therefore unable to sustain efficient UV generation. In someembodiments, the use of said PCF overcomes the limitations of large-core(>25-μm diameter) standard (solid-silica) fibers, which are typicallyunable to generate an output beam in a stable polarization state (>50:1polarized) because of multi-mode operation and/or insufficient in-fiberdiscrimination between orthogonal polarization states, resulting againin inefficient UV generation.

In some embodiments, even higher peak power in the UV (for example, insome embodiments, >500 kW) is enabled by frequency tripling,quadrupling, or quintupling of a source featuring an amplifier thatconsists of a backward-pumped very-large-core (>50-μm diameter) PCR.

In some embodiments, a pulsed optical source featuring an Yb-doped,large-core PCF or very-large-core PCR and generating spectrally narrow,linearly polarized, high-peak-power pulses in the 1.0-1,1-μm wavelengthrange is used to generate light in the deep UV by high harmonicsgeneration. For example, in one embodiment, light in the 167-183-nmwavelength range is obtained by sixth harmonic generation of thefundamental 1.0-1.1-μm pulses, in another embodiment, light in the142-157-nm wavelength range is obtained by 7th harmonic generation; inyet another embodiment, light in the 125-137-nm wavelength range isobtained by 8th harmonic generation. In other embodiments, even shorterwavelengths are obtained by higher harmonic generation. In all of theseembodiments, the efficient generation of optical power useable byapplications such as laser marking and spectroscopy at these shortwavelengths is fundamentally enabled by the high peak power (>1 MW),linear polarization, beam quality, and narrow pulse spectral linewidthof the pulsed fiber-based sources described in this invention, whichpermits the overcoming of limitations in standard fibers. Indeed, noprior art reporting generation of harmonics higher than the fifth fromany fiber-based sources exists to date.

In some embodiments, an optical source featuring single-transverse-mode,single-polarization or polarization-maintaining, Yb-doped PCF or PCRamplifiers is used to generate mid-infrared light in the 1.5-4,0-μmwavelength range by frequency conversion in an optical parametricoscillator (OPO) or optical parametric generator (OPG). In someembodiments, the OPO includes nonlinear crystal (e.g., in someembodiments, periodically-poled lithium niobate (PPLN) or KTP)incorporated in an optical cavity formed by two or more mirrors, asdescribed, for example, in A. Henderson et al., Optics Express 14,767-772 (2006).

In some embodiments, the pulse energy, peak power, and average powerobtained at mid-infrared wavelengths using a source that features alarge-core PCF or very-large-core PCR are higher than obtained bypumping the OPO with a source featuring small-core (less than about25-μm diameter) polarization-maintaining fibers exhibiting peak-powerand polarization shortcomings.

In some embodiments, mid-infrared wavelengths longer than 4 μm aregenerated by using a pulsed optical source that generates light in the1.0-1.1-μm wavelength range, features a large-core PCF orvery-large-core PCR, and is followed by two OPO in series (for example,in some embodiments, a PPLN OPO pumped by said source and generatingsignal or idler beam in the wavelength range 2-3 μm, which is used topump a second OPO featuring, in some embodiments, a zinc germaniumphosphide (ZGP) crystal). In some other embodiments, the optical sourcefeaturing PCF and/or PCR as described above is followed by a single OPOdirectly generating radiation at wavelengths longer than about 4 μm(e.g., in some embodiments, an OPO featuring an optically patternedgallium arsenide (OPGaAs) crystal).

High-Energy, Chirped-Pulse-Amplifier Systems Featuring Large-Core PCF orVery-Large-Core PCR.

In some embodiments, an Yb-doped, large-core PCF or very-large-core PCRis used as the power amplifier in a chirped pulsed amplification (CPA)system. In CPA, low-pulse-energy, ultra-short pulses (e.g., pulsesshorter than 10 ps, in some embodiments) produced by a seeder (e.g., amode-locked solid-state laser in some embodiments) are temporallystretched to a duration of a few nanoseconds using a bulk diffractiongrating pair, chirped fiber Bragg grating, or dispersive delay line(i.e., a piece of un-doped optical fiber) and injected into an amplifierto increase the pulse energy (e.g., up to a few mJ in some embodiments).The pulse-stretching process is instrumental in lowering the peak power,which permits the achieving of high pulse energies without encounteringproblems such as optical damages or spectral degradation. Uponamplification, the pulses are recompressed to their original durationusing another bulk diffraction grating pair or other dispersive opticalassembly capable of sustaining very high peak power without damages. CPAsystems using Yb-doped fiber amplifiers are well documented in theliterature (see A. Galvanauskas, IEEE J. Selected Topics in QuantumElectron. 7, 504 (2001)) and, recently, CPA systems featuring Yb-dopedphotonic-crystal-fiber amplifiers have been reported (see F. Roser etal., Opt. Lett. 30, 2754 (2005)). However, to date, the pulse energyobtained in these fiber-based systems has been <1.5 mJ, limited byspectral degradation caused by optical nonlinearities due to high peakpower. In some embodiments, very-large-core (core diameter >50-μm) PCRsenable scaling of pulse energy to values >1.5 mJ, as higher peak powercompared to previously implemented fibers can be achieved without theonset of parasitic nonlinearities. In some embodiments, asingle-polarization, Yb-doped PCR of core diameter >50 μm is used as theamplifier in the CPA system. In some embodiments, the core diameter ofthe used PCR is between 50 and 100 μm. In some embodiments, the corediameter of the used PCR is >100 μm.

In some embodiments, the very-large-core Yb-doped PCR amplifier used forhigh-pulse-energy amplification of stretched pulses is fusion-spliced toa hollow-core photonic-band-gap (PBG) fiber. The dispersion propertiesof this fiber are tailored to recompress the pulses to their originalduration, while the hollow core permits the obtaining of very high peakpower without incurring optical damages.

In some embodiments, the high-peak-power pulsed output of an Yb-doped,large-core PCF or very-large-core PCR is remotely delivered by means ofa passive delivery fiber. In some embodiments, a hollow-core PBGdelivery fiber of core diameter matching, in some embodiments, that ofthe PCF or PCR, is used to ensure that the pulses travel along anair-filled path, which minimizes optical nonlinearities even if thedelivery fiber is several meters long, thus affording the preservationof spectral properties in the delivered pulses. In some embodiments, theends of the PBG fibers are “connectorized” in such a way that opticalfeedback onto the PCF or PCR facet is minimized.

MW-Peak-Power, mJ-Pulse-Energy, Multi-kHz-Repetition-Rate Pulses fromYb-Doped Fiber Amplifiers

In at least one embodiment, MW-peak-power, mJ-pulse-energy,multi-kHz-repetition-rate pulses from Yb-doped fiber amplifiers areimplemented in a laser. In some embodiments, Yb-doped fiber amplifiersare used to generate high peak powers in pulses of excellentspectral/spatial quality.

Still further, in some embodiments, the inventors implemented a 1-nspulse, Q-switched microchip laser (having a wavelength of 1062 nm)operating at about a 10-kHz pulse-repetition rate (PRR) that seeds(produces “seed” laser pulses each of 1-ns duration that are fed into) adual-stage amplifier featuring a fiber preamplifier as its first stageand a 40-μm-core Yb-doped photonic-crystal fiber (PCF) as the second, orpower-amplifier stage. In some embodiments of this amplifier,diffraction-limited (M²=1.05) pulses are output with each pulse havingabout one-nanosecond duration, about 1.1-millijoule (mJ) energy, about1.1-megawatt (MW) peak power, about 10.2-W average power, a spectrallinewidth of about 9 GHz, negligible nonlinearities, and slopeefficiency of greater than seventy-three percent.

Moreover, in at least one embodiment, the inventors replaced the seedsource with a shorter-pulse (less than 500-ps) microchip laser (1064 nm)having a PRR of about 13.4 kHz and obtained diffraction-limited(M²=1.05), about-450-ps pulses having energy of greater than 0.7 mJ,peak power in excess of 1.5 MW (the highest from a diffraction-limitedfiber source), average power of about 9.5 W, and spectral linewidth ofless than 35 GHz.

In at least one embodiment, the inventors demonstrated a MOPA systemfeaturing a 1-ns pulse Q-switched microchip laser (having a wavelengthof 1062 nm) that seeds a three-stage amplifier featuring a70-μm-core-diameter Yb-doped PCF. In some embodiments, the output fromthis MOPA had beam quality M²˜1.1, pulse energy in excess of 3 mJ, peakpower in excess of 3 MW, average power ˜30 W, spectral linewidth ˜13GHz, and signal-to-noise ratio ˜60 dB.

In at least one embodiment, the inventors demonstrated a MOPA systemfeaturing a 1-ns pulse Q-switched microchip laser (having a wavelengthof 1062 nm) that seeds a three-stage amplifier featuring a1000-μm-core-diameter Yb-doped PCF. In some embodiments, the output fromthis MOPA had beam quality M²˜1.3, pulse energy in excess of 4.3 mJ,peak power in excess of 4.5 MW, average power ˜42 W, spectral linewidth˜20 GHz, and signal-to-noise ratio ˜60 dB.

Very-Large-Core, Single-Mode Yb-Doped Photonic-Crystal Fiber forMulti-MW Peak Power Generation

In some embodiments, a very-large-core, single-mode Yb-dopedphotonic-crystal fiber for multi-MW peak power generation isimplemented. Generating high peak powers in pulsed rare-earth-dopedfiber sources has been traditionally very challenging due to the onsetof nonlinear optical effects, including SRS and SBS, nonlinear phasemodulation, and four-wave mixing (FWM), all of which strongly degradethe spectral brightness. Additional limiting factors are fiber-facetdamage and bulk optical damage.

In some embodiments, scaling the in-fiber peak power beyond the megawattlevel is implemented, while retaining high spatial and spectral quality.In some embodiments, a single-mode, 40-μm-core-diameter, Yb-dopedphotonic-crystal fiber (PCF) is used to generate peak power greater than1.1 MW with M² of about 1 and spectral linewidth of about 10 GHz. Again,in some embodiments, an intrinsically single-mode Yb-doped PCF featuringa core diameter greater than 60 μm is implemented. In some embodiments,the PCF is optimized for generation of multi-MW peak-power,diffraction-limited pulses exhibiting minimal optical nonlinearities,and is used as the final stage in a master-oscillator/fiber-amplifiersystem generating multi-kHz repetition-rate, sub-ns pulses at 1.06-μmwavelength.

Stimulated Brillouin Scattering (SBS) can lead to power limitations oreven the destruction of a high-power fiber-laser system due to sporadicor unstable feedback, self-lasing, pulse compression and/or signalamplification.

In some embodiments, one way to generate output with more controlledattributes is to use a master-oscillator/power-amplifier (MOPA)architecture. The oscillator can be optimized to generate a laser seedsignal having the appropriate characteristics, such as linewidth, andthe power amplifier is used to increase the output power and/or pulseenergy to much higher levels.

In some embodiments, the structure will also simplify the systemconfiguration for using mode-matching techniques to achieve good beamquality for slightly multimode fiber amplifiers. For a rare-earth-dopedmultimode fiber, the signal launch condition plays an important role inmode selection, as described in U.S. Pat. No. 5,818,630, which isincorporated herein by reference.

In some embodiments, rare-earth-doped photonic-crystal fiber is usedwith a core diameter greater than 50 microns for fabricating a pulseamplifier achieving high peak powers greater than 1 MW.

In some embodiments, large-core single-polarization, rare-earth-dopedphotonic-crystal fibers are implemented for generation ofhigh-peak-power (greater than 100-kW), diffraction-limited, linearlypolarized pulses (i.e., for efficient generation of high-power opticalpulses at visible and UV wavelengths, via harmonic generation).

Still, in some embodiments, a single-frequency, near-transform-limited,pulsed master oscillator is used to prevent spectral broadening bycross-talk effects and self-/cross-phase modulation in high-peak-power(greater than 1 MW) fiber amplifiers.

In some embodiments, an optimized design of a high-peak-power fiberamplifier is implemented including: gain staging for ASE suppression, a“pump-block” design featuring a monolithic pump-injection scheme,band-pass filter(s) as isolator(s), epoxy-free connectorizationscheme(s) (i.e., for standard fibers, PCF and rod-like PCF), and arod-like rare-earth-doped PCF power amplifier.

FIG. 1E is a schematic diagram of a compact system 102 having aplurality of high-peak-power rare-earth-doped photonic-crystal-fiber(PCF)/photonic-crystal-rod (PCR) optical power amplifiers. In someembodiments, compact system 102 is conceptually similar to system 101 ofFIG. 1B, but with the addition of a highly reflective mirror 129 in ornext to at least some of the pump blocks 119 between stages, in order toallow side-by-side placement of at least some of the amplifierphotonic-crystal fibers and/or rods for a shorter and more compactconfiguration. Further, in some embodiments, in each pump block 119 thedichroic mirror/beamsplitter 113 is highly reflective for light of thesignal wavelength and highly transmissive for light of the pumpwavelength. In some embodiments, in order to reduce loss of pump powerto parasitic signals, the filter in each pump block 119 reduces opticalASE noise of wavelengths other than the signal wavelength travelingforward, and the isolator in each pump block 119 reduces SBS, ASE andother noise traveling backward. In some embodiments, successiveamplifier stages are larger diameters and shorter lengths to handleincreasing power.

FIG. 1F is a schematic diagram of a compact system 103 having aplurality of high-peak-power rare-earth-doped PCF/PCR optical poweramplifiers. In some embodiments, compact system 103 is conceptuallysimilar to system 102 of FIG. 1E, but eliminates mirror 129 in or nextto the pump blocks 119 between stages, in order to reduce mirror losses,but at the cost of a larger footprint of the amplifier photonic-crystalfibers and/or rods. Other aspects of system 103 are as described abovefor systems 101 and 102 of FIG. 1B and FIG. 1E.

FIG. 1G is a schematic diagram of a pump block 119. In some embodiments,pump block 119 is conceptually similar to pump block 118 of FIG. 1D, butthe dichroic mirror/beamsplitter 123 is highly reflective for light ofthe signal wavelength and highly transmissive for light of the pumpwavelength.

FIG. 1H is a schematic diagram of a compact system 104 havinghigh-peak-power rare-earth-doped PCF/PCR optical power amplifiers. Insome embodiments, compact system 104 is conceptually similar to system103 of FIG. 1F, but reflects the signal at a more acute angle to form asmaller triangular footprint of the amplifier photonic-crystal fibersand/or rods. Other aspects of system 104 are as described above forsystems 101, 102 and 103 of FIG. 1B, FIG. 1E and FIG. 1F.

FIG. 1I is a schematic diagram of a pump block 121. In some embodiments,pump block 121 is conceptually similar to pump block 119 of FIG. 1G, butthe dichroic mirror/beamsplitter 133 reflects at a smaller angle. Inother embodiments, an even smaller angle of reflection is used to allowa bow-tie-shaped compact system similar to system 104 but with afootprint approaching that of system 102, described above. In someembodiments, dichroic mirror/beamsplitter 133 has a parabolic or othershape that focuses the reflected signal, allowing elimination of one ormore of the lenses 112.

A key factor limiting the peak power in pulse fiber amplifiers isoptical damage occurring in the fiber output facet. This part of thefiber is the most vulnerable to optical damages due to the inevitablepresence of microscopic defects and/or contaminants on the facetsurface, which can absorb light, heat up, and eventually release shockwaves resulting in catastrophic cracks. A widely accepted value for thepeak optical intensity at the onset of surface damage in silica is about40 GW/cm² for optical-pulse durations of about 1 ns (see, for example,B. C. Stuart et al., Physical Review B 53, 1749-1761 (1996)). Because ofthe tiny beam area in the fiber core, such optical intensity can bereached at relatively modest pulse peak powers. For example, a pulsepeak power of about 8 kW is sufficient to produce facet damage intelecommunication fibers (e.g., having an in-core beam area of about2×10⁻⁷ cm²) and a pulse peak power of about 350 kW is sufficient toproduce facet damage in the 40-micron core-diameter PCF described inthis document (having an in-core beam area of about 9×10⁻⁶ cm²).Conversely, the threshold intensity for bulk damage within each fiber istypically higher by more than order of magnitude than the above values.

FIG. 2 is a schematic diagram of a high-peak-power rare-earth-dopedphotonic-crystal-rod optical power amplifier 200. In some embodiments, aphotonic-crystal fiber or rod 115C includes an array of parallel holes212 that surround a core 214 that has substantially the same index ofrefraction as the inner cladding 210 surrounding the core. The use ofphotonic-crystal-fiber holes allows more precise control over the indexof refraction, allowing the design of cores having a very low NA, andthus providing optical-amplifier operation using even fibers or rodshaving large core areas in a single lowest-order (LP01) mode, resultingin an output beam with high beam quality for high-power amplifiers. Insome embodiments, the problem of facet damage can be completely orsubstantially eliminated by using a beam-expanding endcap 216. In someembodiments, the photonic-crystal-fiber holes are collapsed (e.g.,melted shut) or filled with an index-matching compound, such that thecore ends at a plane 215, and the signal 296 expands to a larger spot297 on exit facet 218. In some embodiments, facet 218 of endcap 216 isangle-polished (relative to plane 217, which is perpendicular to thebeam axis) to prevent optical feedback, as is done with standard fiberfacets. The beam exiting the doped-fiber portion at plane 215 expands inthe endcap freely by diffraction (i.e., without changes in spatialquality, M²) provided that its diameter remains smaller than the outerdiameter of the endcap. In some embodiments, by expanding the beam, theoptical intensity at the output facet of the endcap is made lower thanthe surface-damage threshold for silica, which avoids damage to theendcap. The magnification factor (defined as the ratio between in-coreand expanded beam diameter) is approximately proportional to the endcaplength. In some embodiments, the endcap is thus made to a lengthproportional to the spot size 297 having the desired beam radius atfacet 218. In some embodiments, a laser-diode pump 116 providesbackward-traveling pump light reflected by dichroic mirror/beamsplitter126 (which, in some embodiments, has a reflective surface that alsofocuses the pump beam into facet 218). Output beam 299 has furtherexpanded to size 298 at dichroic mirror/beamsplitter 126, reducing thepossibility of damage to that component, while retainingdiffraction-limited quality.

FIG. 3A is an end-view schematic diagram of a high-peak-powerrare-earth-doped photonic-crystal rod 300 (PCR 300) having abeam-expanding endcap that can be used in a linearly polarized,diffraction-limited, pulsed fiber amplifier, having a large-coreYb-doped photonic-crystal structure, as described herein. FIG. 3B is aside-view schematic diagram of PCR 300. FIG. 3C is a cross-sectionschematic diagram of PCR 300. Several publications (see, for example, J.Limpert et al., Applied Physics B 75, 477-479 (2002)) describe oneconcept of an endcap in preparing optical-fiber ends.

Referring to FIG. 3B, photonic-crystal fiber 310 has a rare-earth-dopedcore 311 surrounded by long narrow holes (or long narrow regions of asolid having a lower index of refraction or concentric layers ofalternately lower and higher indices of refraction) aligned parallel tothe central axis of the fiber, having sizes and arranged in a spacedpattern that defines the size and numerical aperture of the core thatcontains the signal optical beam 312 (see, e.g., U.S. Pat. No. 6,845,204to Broeng et al., issued Jan. 18, 2005, and U.S. Pat. No. 6,829,421 toForbes et al., issued Dec. 7, 2004, which are both incorporated hereinby reference). In some embodiments endcap 320 is a short piece ofcore-less fiber or rod, fusion-spliced to the output end 309 of fiber310, such that beam 312 exits the core into diffracted beam 322 thatexpands to the size of spot 323 by the time it reaches exit facet 321.In other embodiments, the endcap 320 is a short piece of fiber having acore with a diameter much larger than the rare-earth-doped fiber 310 towhich it is fusion-spliced. In a simple embodiment, this large-corefiber used for endcap 320 is a commercially available undoped multimodefiber, wherein preservation of beam quality is ensured as long as thediameter of the beam 322 expanding in the endcap remains smaller thanthe core diameter of endcap 320. If this condition (or the conditionabove, for coreless endcaps) is not met, the free diffraction of thebeam will be hindered by the optical guidance of the multimode core,which results in beam-quality degradation.

MW-Peak-Power, mJ-Pulse-Energy, Multi-kHz-Repetition-Rate Pulses fromFiber Amplifiers Directly Emitting in the “Eye-Safe” Wavelength Range.

In some embodiments, a pulsed optical beam in the 1.5-1,6-μm and1.8-2,5-μm wavelength ranges, usually referred to as “eye-safewavelengths”, is generated while retaining the same design solutions,advantages, and performance capabilities (including high pulse energyand high peak/average power, narrow-spectrum, and single-transverse-modebeam quality) of optical sources based on Yb-doped PCF and PCR operatingin the 1.0-1,1-μm wavelength range.

In some embodiments, optical sources operating at eye-safe wavelengthsare configured like the MOPA schematically illustrated in FIG. 1A andinclude all of the components of said MOPA, each of which has the samefunctionality, with the provisions that the master oscillator 110 is alaser emitting light of eye-safe wavelength in the 1.5-1,6-μm range,each optical component used in the MOPA is designed for operation at thewavelength emitted by the master oscillator, each amplifier stage in theMOPA is built using Er-doped or Er/Yb-codoped fiber, and one or more ofthe amplifiers, among which the final amplifier 115C, feature alarge-core (e.g., in some embodiments, at least 30-μm diameter) Er-dopedor Er/Yb-codoped, single-transverse-mode PCF.

In some embodiments, the final amplifier 115C features asingle-transverse-mode, very large-core (at least 50-μm diameter)Er-doped or Er/Yb-codoped PCR, which is designed in a similar way to theYb-doped PCR described in previous embodiments.

In some embodiments, Er-doped and Er/Yb-codoped PCFs and/or PCRs featureembedded stress-inducing elements, or in other embodiments, designfeatures that depart from cross-sectional circular symmetry so as toinduce sufficient birefringence in the core, which ensures maintenanceof the polarization state of input light and/or guidance of only onepolarization state, as previously described for Yb-doped PCFs and PCRs,resulting in linearly polarized output with polarization extinction of25:1 or better.

In some embodiments, the peak power emitted by Er-doped andEr/Yb-codoped PCFs or PCRs implemented as amplifiers in MOPA systemsexceeds 300 kW in the wavelength range 1.5-1.6 ml, while the beamquality is inherently single-transverse-mode near-diffraction-limited(M²<1.5) and the pulse spectral linewidth is <50 GHz.

In some embodiments, the peak power emitted by the Er-doped andEr/Yb-codoped PCFs or PCRs exceeds 300 kW in the wavelength range1.5-1.6 ml, while the beam quality is inherently single-transverse-modenear-diffraction-limited (M²<1.5) and the pulse spectral linewidth is<50 GHz, and the output beam is >50:1 polarized.

In some embodiments, the peak power emitted by the Er-doped andEr/Yb-codoped PCFs or PCRs exceeds 500 kW in the wavelength range1.5-1.6 μm, while the beam quality is inherently single-transverse-modenear-diffraction-limited (M²<1.5). In some embodiments, the peak poweremitted by the Er-doped and Er/Yb-codoped PCFs or PCRs exceeds 750 kW inthe wavelength range 1.5-1.6 μm, while the beam quality is inherentlysingle-transverse-mode near-diffraction-limited (M²<1.5). In someembodiments, the peak power emitted by the Er-doped and Er/Yb-codopedPCFs or PCRs exceeds 1 MW in the wavelength range 1.5-1.6 μm, while thebeam quality is inherently single-transverse-modenear-diffraction-limited (M²<1.5).

In some embodiments, the linearly polarized, high-peak-power, spectrallynarrow output of a MOPA systems featuring Er-doped or Er/Yb-codoped PCFsand/or PCRs is used for wavelength conversion in a nonlinear crystal orset of nonlinear crystals.

In some embodiments, the output from Er-doped or Er/Yb-codoped PCF orPCR pulsed amplifiers emitting spectrally narrow, linearly polarizedlight is used to efficiently generate pulsed light in the 750-800-nmwavelength range through second-harmonic generation in a PPLN crystal,(see e.g., A. Galvanauskas et al., Optics Letters 22, 105-107 (1997),and U.S. Pat. No. 6,014,249 “Apparatus and method for the generation ofhigh-power femtosecond pulses from a fiber amplifier” to Fermann et al,which are both incorporated herein by reference) or periodically-poledKTP (PPKTP) crystal (see e.g. Champert et al., Applied Physics Letters78, 2420-2421 (2001)), just to name a few. In all such embodiments, thepeak power in the second-harmonic beam is much higher (>100 kW) thanobtained in any prior art, which is enabled by the use ofphotonic-crystal-fiber technology as explained above.

In some embodiments, visible light in the 500-533-nm wavelength rangeand UV light in the 375-400-nm wavelength range are obtained,respectively, by third- and fourth-harmonic generation of ahigh-peak-power Er-doped or Er/Yb-codoped PCF or PCR amplifier asdescribed above in a nonlinear crystal. In some embodiments, thenonlinear crystal used can be fan-out poled magnesium-oxide (MgO) dopedlithium niobate (LiNbO₃) as shown in K. Moutzouris et al., OpticsExpress 14, 1905-1912 (2006). The peak power obtained in this wavelengthrange is higher than any prior art, thanks to the use of PCF/PCRtechnology.

In some embodiments, efficient higher-harmonic (fifth, sixth, seventh orhigher) generation of Er-doped or Er/Yb-codoped PCF or PCR amplifiers ismade possible by the high peak power, narrow spectral linewidth, andlinear polarization offered by these sources, so as to produce light ofwavelength <375 nm.

In some embodiments, a high-peak-power Er-doped or Er/Yb-codoped PCF orPCR amplifier is used to pump an OPO or OPG featuring PPLN or othercrystal so as to enable generation of light in the 3-4-μm wavelength andachieve higher peak power, pulse energy, and/or average power in thiswavelength range than in any example of prior art concerning OPOs orOPGs pumped by fiber-based sources operating at eye-safe wavelengths.

In some embodiments, the system consisting of OPO/OPG pumped by Er-dopedor Er/Yb-codoped PCF or PCR amplifier features a front-end consisting ofreadily available and reliable components traditionally used intelecommunication applications and represents a key enabling technologyfor lower-cost, higher-power, higher-reliability military infraredcountermeasures.

In some embodiments, a very-large-core Er-doped or Er/Yb-codoped PCR asdescribed above is used as a stretched-pulsed amplifier attaining pulseenergy >0.5 mJ in a CPA system producing pulse duration <10 ps in the1.5-1,6-μm wavelength range.

Embodiments corresponding to all of the previous embodiments of pulsedeye-safe sources can be realized by means of large-core orvery-large-core PCFs and/or PCRs emitting a near-diffraction-limitedoutput (M²<1.5) and doped with thulium (Tm) and/or Holmium (Ho) whichemit light in the 1.8-2,5-μm wavelength range.

Fiber-End Treatment for High-Peak-Power Handling

In some embodiments, beam-expanding endcaps for PCFs are fabricated bythermally collapsing the inner axial channels over a desired length fromthe facet, which destroys waveguidance in the PCF core and pump claddingand, therefore, enables free beam expansion. In some embodiments,controlled thermal collapse of the inner PCF axial channels is obtainedthrough heating with a commercial fusion splicer. The resulting endcapdiameter coincides with that of the PCF overcladding. Note that, in someembodiments, the endcap constitutes also a protective seal (i.e., itcloses the ends of the channels), which enables standard polishing ofthe PCF facets. Without endcaps, it would be impossible to polish a PCFwithout the risk that water, polishing compounds, glass particulate andother contaminants would be attracted into the PCF air-filled innerchannels by capillarity, which would compromise the PCF performance.

Although a similar endcap-fabrication method has been described in theopen literature (see, for example, R. E. Christiansen et al.,Proceedings of the 4th Reunion Espanola of Optoelectronics (OPTOEL2005), CI-5 pp. 37-49), the available prior art pertains to shortendcaps only, the length of which cannot exceed the width of theelectric arc or filament of the fusion splicer (less than about onemillimeter). This limitation hampers beam expansion, especially in thecase of large-core, intrinsically single-mode PCFs. Indeed, thedivergence angle of a near-diffraction-limited beam exiting a fiber isinversely proportional to the mode field diameter, whereas its Rayleighrange is proportional to the mode field area (the Rayleigh range, z_(R),is the distance from the core over which a near-diffraction-limited beamexiting a fiber exhibits negligible divergence and is defined asz_(R)=αn×MFD²/(λM²) where n is the material refractive index, MFD is themode field diameter, and λ is the wavelength). Both properties tend tolower the rate of beam expansion per endcap unit length. For example,the beam-diameter magnification factor provided by a 1-mm-long endcapexceeds 100 for a small-core telecommunication-like fiber (MFD of about5 microns), but is only about 1.5 for the single-mode40-micron-core-diameter PCF described above. Longer endcaps are thenrequired to generate megawatt peak powers without surface damage.Moreover, no prior art has been found that describes methods forfabricating endcaps (of any length) in photonic-crystal rods.

The present invention provides an endcap-fabrication method that enablesendcaps of arbitrary length for photonic-crystal fibers (PCFs) andphotonic-crystal rods (PCRs). In a simple embodiment, this method relieson commercially available fiber fusion splicers only. The methodincludes two steps: tapering and zipping.

As previously described, PCRs are characterized by a large (greater thanabout one mm in diameter) silica overcladding. This overcladding leadsto a considerably larger size and thermal mass compared to standardfibers (e.g., fibers for telecommunications). Therefore, PCRs are notamenable to being processed with standard fusion splicers because (a)their dimensions are incompatible with key splicer features (mainly,electrode spacing or filament size), and (b) higher temperatures and/orlonger fusion times are required to cause the collapse of the innerholes than are available on such splicers.

Some embodiments of the present invention utilize a simple first step toovercome this problem—tapering down the PCR over-cladding near the facetby grinding, lapping, or shaving it on a commercial fiber polisher.

FIG. 3D is an end-view schematic diagram of a high-peak-powerrare-earth-doped photonic-crystal rod 301 partially fabricated intohaving a beam-expanding endcap by applying a protective epoxy cap 308that is later to be removed. FIG. 3E is a side-view schematic diagram ofPCR 301. FIG. 3F is a cross-section schematic diagram of PCR 301. Insome embodiments, before and during the tapering, the temporary epoxycap 308 is applied to the PCR's end 316 to seal the PCR holes andprevent contamination during grinding or polishing. PCR 301 starts witha rod 310 having photonic-crystal-rod holes 311, the ends of which aresealed with cap 308, e.g., a polymer such as epoxy. In some embodiments,a recommended material for the epoxy cap is Crystalbond™, an inexpensiveand residue-free adhesive of high viscosity and extremely short (lessthan ten seconds) hardening time. One current source is ElectronMicroscopy Sciences, P.O. Box 550, 1560 Industry Road, Hatfield, Pa.19440; also at(www.emsdiasum.com/microscopy/products/materials/adhesives.aspx?mm=9).

FIG. 3G is an end-view schematic diagram of a high-peak-powerrare-earth-doped photonic-crystal rod 302 partially fabricated intohaving a dispersing endcap after tapering. FIG. 3H is a side-viewschematic diagram of PCR 302. FIG. 3I is a cross-section schematicdiagram of PCR 302. Some embodiments use a commercial rough, waterproofsanding pad (e.g., 220 grit SiC sandpaper) mounted to a standard rotaryfiber polisher. The taper 315 reduces the over-cladding diameter to avalue compatible with fusion splicers over a suitably short portion ofthe rod near the facet, and also does not compromise the rigidity of therod and, therefore, does not introduce bend loss.

FIG. 3J is a microphotograph of PCR 304 after tapering the end andbefore removing the epoxy cap and collapsing the air cladding to formthe beam-expanding endcap. In some embodiments, the epoxy cap is removedby breaking the rod 2-3 mm from the PCR facet 318.

In some embodiments, a second step includes piece-wise thermalcollapsing of cladding holes (“zipping”). The fabrication of endcaps ofarbitrary length on PCFs or end-tapered PCRs (both hereafter referred toas “the fiber”) is illustrated in FIGS. 7A, 7B, 7C, and 7D.

In some embodiments, the method includes the following operations: (a)positioning the fiber tip into the heating zone of the splicer, i.e.,the space between the electrodes in an arc splicer or the filament ovenin a filament splicer, respectively; (b) running a fusion splicing cyclethat provides sufficient heat to collapse the fiber inner channels inthe heating zone—this operation forms a preliminary endcap (In someembodiments, the heating process during the fusion cycle is controlledby setting appropriate values for the current in the arc discharge (orin the filament) and fusion time. Setting these parameters is a standardprocedure in the use of commercial fusion splicers. In some embodiments,the amount of heat supplied to the fiber should be just enough tocollapse the channels, not more. Excessive heating may result in bendingof the fiber tip.); (c) feeding the fiber through such that the inneredge of the preliminary endcap approximately corresponds to the edge ofthe heating zone (in some embodiments, this operation can be executedvery accurately by using the electronically controlled step motor thatpositions the splicer holder (in manual setting) and the splicer displayas the monitor); and (d) running the fusion cycle again—the innerchannels will zip up inward over the width of the heating zone, thusextending the existing endcap. Steps (c) and (d) can be repeated anarbitrary number of times to obtain the desired endcap length.

FIG. 4A is a side-view schematic diagram of a tapered high-peak-powerrare-earth-doped photonic-crystal rod 401 partially fabricated intohaving a beam-expanding endcap after tapering and removing the epoxycap. In the embodiments of FIGS. 4A-4D, the taper 315 is formed beforecollapsing or filling the holey region of the endcap, e.g., by meltingthe rod or by filling with an index-matching substance. In someembodiments, a vacuum is applied via the far end of the holey region toassist the collapsing/filling. At this point, the holey region end 411is at the rod end.

FIG. 4B is a side-view schematic diagram of a high-peak-powerrare-earth-doped photonic-crystal rod 402 after partiallycollapsing/filling the holey region of the endcap. At this point, theholey region end 412 is a short distance in from the rod end.

FIG. 4C is a side-view schematic diagram of a high-peak-powerrare-earth-doped photonic-crystal rod 403 after furthercollapsing/filling the holey region of the endcap. At this point, theholey region end 413 is a further distance in from the rod end.

FIG. 4D is a side-view schematic diagram of a high-peak-powerrare-earth-doped photonic-crystal rod 404 after yet furthercollapsing/filling the holey region. At this point, the holey region end414 is slightly further than the desired distance from the rod end toallow for removal of material during the angle-polishing process of therod end 318 to form end facet 319.

FIG. 4E is a side-view schematic diagram of a high-peak-powerrare-earth-doped photonic-crystal rod 404 with a finished endcap 416having end facet 319. At this point, a sufficient amount of material isremoved from the end facet 319 during the angle-polishing process suchthat the holey region end 414 is at its final position and desireddistance from the polished rod end.

FIG. 4F is a side-view schematic diagram of a high-peak-powerrare-earth-doped photonic-crystal fiber or rod 405 with a beam-expandingendcap formed by an index-matching compound drawn into the air cladding311. The index-matching compound creates a non-guiding region 323 in thefiber/rod and allows the beam within the core to freely expand. Thefiber/rod does not need to be heated and sealed by an arc from a fusionsplicer and is therefore not tapered. The compound seals the aircladding, which allows angle-polishing of the fiber/rod end.

FIG. 5A is a side-view schematic diagram of a high-peak-powerrare-earth-doped photonic-crystal fiber (PCF) 500 after collapsing theholey region of the endcap and angle-polishing the end facet 319. Insome embodiments, the outer dimension of the PCF is small enough (or thefusion splicer is large enough) that the fiber can be placed into thefusion splicer without tapering, and so no tapering need be done. Insome embodiments, polishing of the fiber facet 319 is performed aftercollapsing the holey region, such that the collapsed region seals therod's end and prevents contamination of the inner holey region 311further down the rod 500 during polishing the facet 319.

FIG. 5B is a microphotograph of PCF 500 having an endcap fabricated withthe above method on a 40-μm-core PCF. This microphotograph correspondsto the diagram of FIG. 5A.

FIG. 6 is a microphotograph of PCR 600 after collapsing the holey regiona sufficient length and angle-polishing the endcap of a 70-μm-core PCR.These endcaps are the longest ever obtained in photonic-crystal fibers.In particular, the endcap on the PCR is the first obtained for this typeof fiber.

FIG. 7A is a side-view block diagram of a system 700 for forming abeam-expanding endcap onto high-peak-power rare-earth-dopedphotonic-crystal rod 310 at a time before collapsing the holey region ofthe endcap. In some embodiments, the heating apparatus 710 is aconventional fiber-fusion or fiber-splicing device. PCF or PCR 310 isinserted into the hole of heating apparatus 710. In some embodiments,heating apparatus 710 is oriented such that the axis of its hole isvertical, in order to prevent any bending of the fiber or rod beingprocessed. In some embodiments of FIGS. 7A-7G, a vacuum is applied tothe heating apparatus 710 and/or the fiber or rod 310 to prevent leavingany air bubbles after collapsing the holes. In some embodiments, theheating apparatus is flushed with argon to provide an inert environmentand prevent contamination during the collapsing process.

FIG. 7B is a side-view block diagram of a system 700 at a time afterapplying heat and collapsing the end portion of the holey region of theendcap. FIG. 7C is a side-view block diagram of a system 700 at a timeafter moving the endcap further into the heating region. FIG. 7D is aside-view block diagram of a system 700 at a time after furthercollapsing the end portion of the holey region of the endcap.

FIG. 7E is a side-view block diagram of photonic-crystal rod 310 at atime after collapsing the holey region 311 of the endcap 416 andangle-polishing the end 318.

FIG. 7F is an end-view block diagram of system 700 having a circularhole 711, used for some embodiments of FIG. 7A, that is useful forcollapsing the holey region 311 of a PCR 310 that is cylindrical orotherwise fits within hole 711.

FIG. 7G is an end-view block diagram of a system 701 for forming abeam-expanding endcap onto ribbon-like high-peak-power rare-earth-dopedphotonic-crystal rod. In some embodiments, system 701 includes a heatingblock 720 having a hole 721 that is substantially wider than its height,in order to effectively heat a photonic-crystal ribbon such as shown inFIG. 8A.

FIG. 8A is a cross-section-view schematic diagram of a system 800 havinga ribbon-like high-peak-power rare-earth-doped photonic-crystal rod(also called a PCR ribbon) 801. In some embodiments, system 800 includesa heat sink 852 surrounding a double-clad multi-core ribbon 801, whichhas an outer cladding 853 surrounding inner cladding 854 to contain pumplight inside portion 862 so that the pump light can enter the pluralityof side-by-side cores 856, each of which is surrounded by a holey region857. In some embodiments, each holey region 857 is formed with an outershape (such as a plurality of hexagons, one surrounding each core 856)that allows pump light to enter the cores from all sides while alsoreducing the lateral distances between the cores and the heatsinkmaterial 852.

FIG. 8B is a perspective-view schematic diagram of a ribbon-likehigh-peak-power rare-earth-doped PCR ribbon spectral-beam-combinerpower-amplifier output-stage system 808. In some embodiments, system 808includes a PCR ribbon 801 that is end-pumped by pump light 814reflecting off dichroic mirror/beamsplitter 813 through a lens array 812into facet 818 of PCR ribbon 801. In some embodiments, PCR ribbon 801includes an endcap 815 where the holey regions 857 are collapsed orfilled, in order that the output beams expand before they reach facet818 to prevent damage to the facet by their high-intensity opticalradiation. In some embodiments, the output beams 891, 892, through 893are of successively shorter wavelengths and are formed into parallelbeams by lens 812 (e.g., a doubly telecentric lens), and thus diffractoff first diffraction grating 831 at successively different angles toform a group of converging beams 895, and then all reach a singleoverlapped spot on second diffraction grating 832 (which is parallel to,and has a grating pattern substantially identical to, grating 831),wherein the respective wavelengths and core-to-core spacings areselected such that all beams converge to a single spot on grating 832.All of the beams diffract off grating 832 in one spectrally combinedoutput beam 899. In some embodiments, each beam 891-893 is of a narrowlinewidth; however, the second grating 832 removes linewidth spreadingintroduced by the first grating 831. In some embodiments, system 808forms the output power amplifier for a plurality of master oscillatorsas described above for FIG. 1B, wherein each master oscillator iswavelength-tuned to a respective wavelength suitable for spectralcombining by grating 831, grating 832, the grating spacing, and thecore-to-core spacing.

Some embodiments of the present invention optionally include certainaspects disclosed in U.S. patent application Ser. Nos. 11/342,336,titled “APPARATUS AND METHOD FOR SPECTRAL-BEAM COMBINING OF HIGH-POWERFIBER LASERS” (which issued as U.S. Pat. No. 7,199,924 on Apr. 3, 2007)and No. 11/342,337, titled “METHOD AND APPARATUS FOR SPECTRAL-BEAMCOMBINING OF HIGH-POWER FIBER-LASERS” (which issued as U.S. Pat. No.7,233,442 on Jun. 19, 2007), which are hereby incorporated by referencein their entirety.

FIG. 8C is a plan-view schematic diagram of amaster-oscillator/power-amplifier (MOPA) high-peak-powerrare-earth-doped photonic-crystal-ribbon laser system 870 usingPCR-ribbon power-amplifier output-stage system 808. In some embodiments,system 808 includes a PCR ribbon 801 power amplifier having a pluralityof parallel side-by-side photonic-crystal-fiber cores 857, alens/focusing unit 836 that forms a plurality of parallel beams 891-893of wavelengths λ₁-λ_(N) (which, in some embodiments, are circularGaussian beams overlapped one to another such as shown in schematicpattern 839), which are spectrally combined into a single output beam899 by the spectral-beam combiner 873 formed by parallel gratings 831and 832. In some embodiments, lens unit 836 is a telecentric lens havinga very long focal length (e.g., infinite) to the right.

In some embodiments, master oscillator 871 also includes aphotonic-crystal ribbon 874 having a partially reflecting grating 872 atits left end, and a spectral-beam combiner (gratings 821 and 822) andback mirror 823 at its right end to form a lasing cavity for a pluralityof different-wavelength beams of wavelengths λ₁-λ_(N). In someembodiments, the distance between the two gratings 821 and 822 can beincreased in order to narrow the linewidth of each lasing beam, and/ormirror 823 can be placed at a suitable position (e.g., further away) andsized (e.g., smaller radius) to select a suitably narrow linewidth (witha suitable adjustment to the core-to-core spacings of thephotonic-crystal waveguides 857). In some embodiments, lens unit 875 isa telecentric lens having a very long focal length (e.g., infinite) tothe right. In some embodiments, paraboloid-mirrormagnifiers/demagnifiers 876 and 877 are each partial (e.g., half)paraboloids, and are used for two-dimensional enlarging of the parallelbeam pattern 827 to form parallel beam pattern 828 left-to-right andtwo-dimensional shrinking of the parallel beam pattern 828 to formparallel beam pattern 827 right-to-left. In some embodiments, the beamsat the end of PCR ribbon 874 are circular Gaussian beams separated onefrom another such as shown in schematic pattern 827, and the parallelbeams between partial paraboloid 876 and grating 821 are circularGaussian beams non-overlapped one to another such as shown in schematicpattern 828. In some embodiments, the combined feedback beam 829includes a plurality of side-by-side partially combined component beamsthat together are of a different height than the height 889 of theoutput beam 899. In some embodiments, the photonic-crystal waveguides857 of PCR ribbon 874 and the photonic-crystal waveguides 857 of PCRribbon 801 are of very low NA (which would have high bending losses) inorder to support a single mode (e.g., LP01), but are connected to eachrespective other by connection 878 formed of one or more ribbons and/orindividual fibers of a much higher NA and much lower bending losses, inorder to provide a more compact form factor. In some embodiments, thePCR ribbons are end-pumped as shown in FIG. 8B. In some embodiments, theconnection 878 also includes optical isolators and/or narrow band-passfilters (BPFs).

FIG. 8D is a cross-section-view schematic diagram of a preform 861configured to compensate for lateral shrinkage. In some embodiments,preform 861 is heated to a melted or partially melted state, and amulticore ribbon is then pulled to a final desired cross-section andlength. The lateral shrinkage naturally occurs due to surface tension ofthe melted glass when ribbon 862 (see FIG. 8E) is pulled from thepreform 861. The ribbon-like high-peak-power rare-earth-doped-corephotonic-crystal-rod 862 (herein also called ribbon-rod 862) includespump-cladding 865 and core portions 868. In some embodiments, ribbon-rod862 is sandwiched between heatsink portions 852 (see FIG. 8A). Referringagain to FIG. 8D, in some embodiments, each preform core 866 and thesurrounding photonic-crystal hole pattern 867 is wider than high in aratio (e.g., empirically derived) that, once the final ribbon is pulledfrom the melting preform, the pulling process will reshape into thedesired final shape and ratio (e.g., circular cross-sectioned cores), asshown in FIG. 8E. The portion 864 of the preform 861, when pulled, formsthe pump cladding 865 (i.e., the region into which the pump light is tobe injected) of ribbon-rod 862.

FIG. 8E is a cross-section-view schematic diagram of a high-peak-powerrare-earth-doped-core photonic-crystal ribbon-rod 862. In someembodiments, an outer cladding is added to the outer edge such that aheatsink can be added without disrupting pump-light flow, while in otherembodiments, no outer cladding is used (e.g., air (for terrestrial,naval or aircraft uses) or a vacuum (for spaceship uses) forms the outercladding).

FIG. 8F is a cross-section-view schematic diagram of a polarizing PCF orPCR 880 having a single core 868 having one or more stress rods 882 ontwo opposite sides of the core to induce birefringence to preferentiallysupport a single polarization state. In some embodiments, a pumpcladding 865 carries the pump light down the length of the fiber, whichenters the core 868 along its entire length, and an optional outercladding 863 keeps the pump light inside its inner radius.

FIG. 8G is a cross-section-view schematic diagram of a polarizing PCF orPCR ribbon (or ribbon-like high-peak-power rare-earth-dopedphotonic-crystal-rod) 881 having a plurality of separate cores 868 eachhaving one or more stress rods 882 on two opposite sides of each core tointroduce a non-linearity to preferentially support a single linearpolarization mode in each core. In some embodiments, a pump cladding 865carries the pump light down the length of the fibers, which enters thecores 868 along their entire length, and an optional outer cladding 883keeps the pump light inside its inner surface.

FIG. 8H is a cross-section-view schematic diagram of ribbon PCR 887having generally planar stress-inducing regions 886 that run the lengthof the ribbon PCR 887 in order to induce birefringence andsingle-polarization behavior in cores 868.

FIG. 8-I is a cross-section-view of a central portion of the cleaved endof a high-peak-power rare-earth-doped photonic-crystal rod (PCR) 890,according to some embodiments of the invention. In some embodiments, thecore diameter, denoted here as CR, is 100 microns, and the pump-claddingdiameter, denoted here as PUMP CDG is 280 microns. The core diameter ismade large in order to spread the high-power optical pulse over a largerarea to reduce the power density, and thus reduce the optical damage tothe core from high-power pulses. The NA of the core is kept very low(e.g., NA=0.01 to 0.02, in some embodiments) in order to support only asingle low-order mode (e.g., LP01) in the large-mode-area core 868, inorder to shorten the interaction length of the high-power pulse with thecore material (e.g., silica, in some embodiments), which reduces NLEs.The ratio of cladding diameter to core diameter is kept small in orderto force more pump light into the core over a shorter distance. Pumpcladding 865 carries the pump light down the length of the fiber, whichenters the core 868 along its entire length, and a large-diameter outercladding 863 keeps the pump light inside its inner radius, as well asproviding substantial stiffness to the PCR 890. In some embodiments, theouter diameter of outer cladding 863 is well over 1 mm, to providestiffness. In some embodiments, the rod has an outer cladding glasshaving a high Young's modulus value selected to increase stiffness ofthe rod.

In some embodiments, the core NA is 0.09 or less and larger than zero.In some embodiments, the core NA is 0.08 or less and larger than zero.In some embodiments, the core NA is 0.07 or less and larger than zero.In some embodiments, the core NA is 0.06 or less and larger than zero.In some embodiments, the core NA is 0.05 or less and larger than zero.In some embodiments, the core NA is 0.04 or less and larger than zero.In some embodiments, the core NA is 0.03 or less and larger than zero.In some embodiments, the core NA is 0.02 or less and larger than zero.In some embodiments, the core NA is 0.015 or less and larger than zero.In some embodiments, the core NA is 0.01 or less and larger than zero.In some embodiments, the core NA is 0.008 or less and larger than zero.In some embodiments, the core NA is 0.005 or less and larger than zero.

Some embodiments obtain the following performance: a pulse duration of 1ns, obtain a peak power of 4.5 MW, a pulse energy of 4.3 mJ, an averagepower of 40 W, a spectral linewidth of 20 GHz, and a beam-quality M²value of 1.3. In some embodiments, the following design features of a100-micron-core, Yb-doped, photonic crystal rod 890 are instrumental toachieve this performance: the number of holes removed from the nativehexagonal array to produce the core is 19, the core diameter is 100microns, the diameter of each cladding hole is 5.88 microns, the spacingbetween holes (also known as the hole pitch) is 17.3 microns, thediameter-to-pitch ratio is 0.34, the core NA is less than 0.02, thepump-cladding diameter is 280 microns, the diameter of the outercladding 863 (which is made of pure glass) is 1490 microns, the Ybconcentration in the core is 3.0×10²⁵ m⁻³ (corresponding to 976nm-wavelength pump absorption of about 33 dB/m), the rod material arefused silica with co-dopants of Fluorine and Aluminum (used in the coreonly), and the rod is about one meter long. In some embodiments, the PCR890 is available custom made by Crystal Fibre (Crystal Fibre A/S,Blokken 84, DK-3460 Birkerod, Denmark). Note that as far as theinventors know, this PCR is the largest-core fiber ever to exhibitsingle-transverse-mode operation and the fiber with the lowest core NAever. In some embodiments, the fiber is end-pumped with a pumpwavelength of 976 nm, an incident pump power of about 78 Watts (this isthe pump power towards the end facet), and a launched pump power ofabout 67 Watts (this is the pump power that gets into the fiber aftercoupling losses at the end facet).

In other embodiments, PCR 890 is another PCR having core diameter of 70microns that the inventors have used. From this 70-micron-core PCR, thefollowing performance was obtained: a pulse duration of 1 ns, a peakpower of 3.0 MW, a pulse energy of 3.1 mJ, an average power of 29 W, aspectral linewidth of 13 GHz, and a beam-quality M² value of 1.1. Insome embodiments, the following design features of a 70-micron-core,Yb-doped, photonic crystal rod 890 are used to achieve this performance:number of holes removed from the native hexagonal array to produce thecore is 19, the core diameter is 70 microns, the diameter of eachcladding hole is 1.68 microns, the spacing between holes (also known asthe hole pitch) is 12.0 microns, the diameter-to-pitch ratio is 0.14,the core NA is less than 0.02, the pump-cladding diameter is 197microns, the diameter of the outer cladding 863 (which is made of pureglass) is 1650 microns, the Yb concentration in the core is 3.0×10²⁵ m⁻³(corresponding to 976 nm-wavelength pump absorption of about 33 dB/m),the rod material are fused silica with co-dopants of Fluorine andAluminum (used in the core only), and the rod is about one meter long.In some embodiments, the fiber is end-pumped with a pump wavelength of976 nm, an incident pump power of about 55 Watts, and a launched pumppower of about 45 Watts.

FIG. 9A is a schematic diagram of a multi-stage amplifier PCF or PCRribbon MOPA laser system 900. In some embodiments, system 900 includes aPCF or PCR ribbon 915 having a plurality of parallelphotonic-crystal-fiber cores 917 in a monolithic substrate 910 toprovide a plurality of amplification stages. In some embodiments, theseed-laser signal from master oscillator 110 is injected through an endof one of the corner reflectors 929, through band-pass filter (BPF) 914at the near end (in some embodiments, each one of the BPFs 914 serve aplurality of side-by-side beams) and into a first photonic-crystal-fibercore 917 (e.g., one just to left of the center, in order that thehighest power is in the cores at the outside edges, for better heatdistribution). After exiting the far end of the firstphotonic-crystal-fiber core 917, the beam passes through dichroicmirror/beamsplitter 913 (which, in some embodiments, serves a pluralityof side-by-side beams, and is used to inject counter-propagating pumplight into each of the left-hand cores), reflects twice from the uppercorner reflector 929 and after going through the far-end BPF 914,re-enters into a second photonic-crystal-fiber core 917 (e.g., one justto the right of the center). After exiting the near end of the firstphotonic-crystal-fiber core 917, the beam passes through thelower-right-hand dichroic mirror/beamsplitter 913 (used, in someembodiments, to inject counter-propagating pump light into a pluralityof side-by-side beams, one into each of the right-hand cores), reflectstwice from the lower corner reflector 929 and after going through thenear-end BPF 914, re-enters into a third photonic-crystal-fiber core 917(e.g., one just to the left of the first core 917). In some embodiments,after a plurality of such round trips through successive cores, the beamexits system 900.

FIG. 9B is a plan-view schematic diagram of MOPA laser system 900, asdescribed above.

FIG. 9C is an elevation-view schematic diagram of MOPA laser system 900,as described above.

FIG. 10 is a plan-view schematic diagram of a multi-segment MOPA lasersystem 1000 having a segmented final gain section with a plurality ofsegments connected using fiber splices. In some embodiments, system 1000includes a compound PCF or PCR 1015C having a plurality of successivephotonic-crystal-fiber (PCF) or photonic-crystal-rod (PCR) segments1015C1, 1015C2, 1015C3, to provide a plurality of amplification segments(e.g., for configurations that achieve a smaller footprint). In someembodiments, the seed-laser signal from master oscillator 110 isamplified by optical preamplifier 115A, and then passes throughconnector 111 into pump block 118 including dichroic beamsplitter (DBS)mirror 113 and band-pass filter (BPF) 114 at the lower-right end andinto the core of a first photonic-crystal-rod segment 1015C1. Afterexiting the far end of the core of first photonic-crystal-rod segment1015C1, the beam passes through a curved pigtail 1020 into the core ofsecond photonic-crystal-rod segment 1015C2, then through a curvedpigtail 1020 into the core of final photonic-crystal-rod segment 1015C3,and then out through pump block 1018.

In some embodiments of the system shown in FIG. 10, more than one middlesegment 1015C2 is provided, each linked by a respective bridge fiber1020 to the next, while, in other embodiments, the middle segment 1015C2is omitted and the first segment 1015C1 is connected to the finalsegment 1015C3 using a bridge fiber 1020. The embodiment shown in FIG.10 has the bridge fibers providing a bend of about ninety degrees;however, other embodiments use different bend angles (e.g., 180-degreebends to connect parallel PCF segments or PCR segments).

In some embodiments, one or more pump lasers feed pump light in acounter-propagating direction using a DBS mirror 113 to reflect pumplight from a pump 116 into the output end of PCR 1015C1 while the outputsignal passes through that DBS mirror 113. In some embodiments, eachpair of segments 1015C1-1015C2 and 1015C2-1015C3 is connected by weldinga bridge fiber 1020, i.e., a short (e.g., in some embodiments, anapproximately 1-cm or shorter) bendable piece of low-bend-loss or highNA fiber that is fusion spliced at its ends to two successive segmentsof the PCR 1015C. The bridge fiber 1020, in some embodiments, is adouble-clad passive (undoped) fiber featuring core diameter equal to thePCR core diameter, a core NA high enough to sustain bending without loss(e.g., NA of about 0.1), and cladding diameter equal or larger than thePCR pump cladding. Because of the relatively high NA of the bridge-fibercore 1022, the fiber 1020 can sustain multiple transverse modes, in someembodiments. However, the fundamental mode of the PCR 1015C exhibitshigh spatial overlap with transverse modes of the bridge fiber 1020 thatfeature a central maximum (for example, the fundamental mode) and,therefore, correspond to good beam quality. In some embodiments, giventhe very short length of the bridge fiber, parasitic inter-modalcoupling (mode scrambling) is negligible. For these reasons, the signalfield propagating in the bridge fiber 1020 does not degrade its spatialquality significantly compared to that in the PCR segments of compoundPCR 1015C. As a result, despite the core NA mismatch at the splice 1021between the bridge fiber 1020 and successive segments of compound PCR1015C, the splice loss remains low. In some embodiments, a suitablysmall physical footprint or a particular desired shape is achieved usingthe folding obtained by the bridge fiber(s). In some embodiments, thebottom sides of two or more of the plurality of segments of compound PCR1015C are laser-welded to a suitable monolithic substrate (such as aglass substrate that is compatible with the glass of the PCR 1015C, andto which all the relevant components are laser-welded) for physicalstability (for example, see the configuration of substrate 1155 of FIG.11B). In some embodiments, the laser-welded assembly is placed and/orsealed in a housing (e.g., similar to housing 109 of FIG. 1C).

FIG. 11A is a perspective-view schematic diagram of a high-peak-powerrare-earth-doped laser-welded PCF or PCR monolithic MOPA laser system1100. System 1100 is similar in some ways to system 1000 of FIG. 10,except that the bridge fiber connections are replaced by a free-spaceoptical coupling, e.g., using micro lenses 1151 and reflecting prisms1152, in some embodiments. In some embodiments, the bottom sides of twoor more of the plurality of segments (1114C1, 1114C2, 1114C3) ofcompound PCR 1114C are laser-welded to a suitable monolithic substrate1155 (such as a glass substrate that is compatible with the glass of thecompound PCR 1114C) for physical stability. In some embodiments, thesides of two or more of the plurality of segments of compound PCR 1114Care laser-welded to each other (see FIG. 11C). In some embodiments, thelaser-welded assembly 1100 is placed and/or sealed in a housing.

In some embodiments of the systems shown in FIGS. 11A to 11-I, more thanone middle segment (e.g., 1114C2) is provided, each related to the nextby the optical components shown in the respective figures, while, inother embodiments, the middle segment (e.g., 1114C2) is omitted and thefirst segment (e.g., 1114C1) is optically connected to the final segment(e.g., 1114C3) by the optical components shown in the respectivefigures.

FIG. 11B is a side-elevation-view schematic diagram of MOPA laser system1100 as seen along cut-line 11B of FIG. 11A.

FIG. 11C is an end-view schematic diagram of MOPA laser system 1100 asseen along cut-line 11C of FIG. 11A.

FIG. 11D is a perspective-view schematic diagram of anotherhigh-peak-power rare-earth-doped laser-welded PCF or PCR monolithic MOPAlaser system 1101. System 1101 is similar in some ways to system 1100 ofFIG. 11A, except that one or more of the micro lenses 1151 are replacedby integrated lens caps 1153 that perform the focusing function for thefree-space optical coupling that, for example, also uses reflectingprisms 1152, in some embodiments. In some embodiments, the lens caps1153 are polished directly on the ends of the component segments (firstsegment 1115C1, optionally one or more intermediate segments 1115C2, andfinal segment 1115C3) of compound PCR 115C, while in other embodiments,lenses are laser-welded (similar to the configuration shown in FIG. 12C,but with the lenses laser-welded or fused onto the ends of componentsegments (1115C1, 1115C2, 1115C3) of compound PCR 115C). In someembodiments, the lens caps 1153 are configured to perform the focusingfunction needed to couple the signal and/or pump light across thefree-space gap between the segments of compound PCR 115C and prisms1152. In some embodiments, the bottom sides of two or more of theplurality of segments of compound PCR 115C are laser-welded to asuitable monolithic substrate 1155 (such as a glass substrate that iscompatible with the glass of the PCR 115C) for physical stability. Insome embodiments, the sides of two or more of the plurality of segmentsof compound PCR 115C are laser-welded to each other (see FIG. 11F). Insome embodiments, the laser-welded assembly 1101 is placed and/or sealedin a housing.

FIG. 11E is a side-elevation-view schematic diagram of MOPA laser system1101 as seen along cut-line 11E of FIG. 11D.

FIG. 11F is an end-view schematic diagram of MOPA laser system 1101 asseen along cut-line 11F of FIG. 11D.

FIG. 11G is a perspective-view schematic diagram of yet anotherhigh-peak-power rare-earth-doped laser-welded PCF or PCR MOPA lasersystem 1102. System 1102 is similar in some ways to system 1101 of FIG.11D, except that one or more of the refracting lens caps 1153 and prisms1152 are replaced by integrated angled reflecting focusing-mirrorendcaps 1154 that perform the focusing function for the internal opticalcoupling that, for example, also uses flat reflecting prism endcaps1155, in some embodiments, each of which is formed directly on the endsof PCR segments (1116C1, 1116C2, 1116C3) of compound PCR 1116C. Thefocusing endcaps 1154 and/or flat endcaps 1155 are reflection coated(e.g., using high-efficiency layered dielectric coatings, in someembodiments) to form an optical path between the ends of successive PCRcores. In some embodiments, the transverse cross sections of PCRsegments (1116C1, 1116C2, 1116C3) of compound PCR 1116C have flat sides(e.g., a rectangular (as shown in FIG. 11-I), hexagonal, octagonal, orother suitable shape) such that when laser-welded together (or, in otherembodiments, connected by other material such as index-matchingtransparent plastic or oil or gel), a continuous internal optical pathis formed between the ends of successive PCR cores. In some embodiments,the bottom sides of two or more of the plurality of segments of compoundPCR 1116C are laser-welded to a suitable monolithic substrate 1155 (suchas a glass substrate (as shown in FIG. 11E) that is compatible with theglass of the PCR 1116C) for physical stability. In other embodiments,only the sides of segments of compound PCR 1116C are welded together,and substrate 1155 is omitted (as shown in FIG. 11H and FIG. 11-I). Insome embodiments, the sides of two or more of the plurality of segmentsof compound PCR 1116C are laser-welded to each other (see FIG. 11-I) formechanical stability and to form a continuous internal optical path. Insome embodiments, the laser-welded assembly 1100 is placed and/or sealedin a housing.

FIG. 11H is an elevation-view schematic diagram of MOPA laser system1102.

FIG. 11-I is an end-view schematic diagram of MOPA laser system 1102.

FIG. 12A is a schematic diagram of a high-peak-power rare-earth-dopedPCF or PCR MOPA laser system 1200 having an improved delivery fiber1230. In some embodiments, pump laser 116 provides pump light into theoutput end of PCR 115C through pump block 1218 that includes a dichroicmirror/beamsplitter 113 (see pump block 1018 of FIG. 10). In someembodiments, delivery fiber 1230 includes an input connector endcap 1220or 1222, and/or an output connector and/or endcap 1210. In someembodiments, delivery fiber 1230 is a hollow-core photonic-crystal (PC)fiber (also called a photonic-bandgap fiber). Further detaileddescription is provided below.

FIG. 12B is a cross-section-view schematic diagram of an output endcap1210 of improved delivery fiber 1230. In some embodiments, a tube 1214(e.g., a glass ferrule tube, in some embodiments) is laser-welded (joint1215) at one end to the output end of hollow-core PC delivery fiber 1230and at the other end to output window 1219, in order to seal outcontaminants from the PCF holes and hollow core of PC delivery fiber1230. In some embodiments, output window 1219 is angled andanti-reflection coated at its inner and/or outer surfaces, in order toreduce detrimental reflections. In some embodiments, the length of tube1214 is sufficient such that the length of inner chamber 1216 allowssome spreading of the output-signal beam, in order to reduce the powerdensity as the beam encounters window 1219 and thus reduce opticaldamage to window 1219 and window surfaces 1218 at high beam powers. Insome embodiments, a threaded ferrule connector 1223 (see FIG. 12C) isalso included around tube 1214, in order to connect to an externalsystem component.

FIG. 12C is a cross-section-view schematic diagram of an input end 1220of improved delivery fiber 1230. In some embodiments, input end 1220includes a tube 1224 (e.g., a glass ferrule tube, in some embodiments)that is laser-welded at one end (joint 1225) to the input end ofhollow-core PC delivery fiber 1230 and at the other end to lens 1229, inorder to seal out contaminants from the PCF holes and hollow core of PCdelivery fiber 1230. In some embodiments, lens 1229 is anti-reflectioncoated at its inner and/or outer surfaces, in order to reducedetrimental reflections. In some embodiments, the length of tube 1224 issufficient such that the length of inner chamber 1216 allows lens 1229to focus the output-signal beam into the hollow core of delivery fiber1230.

FIG. 12D is a cross-section-view schematic diagram of an alternativeinput end 1222 of improved delivery fiber 1230. In some embodiments,input end 1222 includes a shaped sealed endcap (in order to seal outcontaminants from PCF holes and the hollow core of PC delivery fiber1230, and to focus the output signal beam) formed from collapsing thePCF holes and hollow core of PC delivery fiber 1230-or fusion splicingthe photonic bandgap fiber to a coreless fiber with a suitable diameter,and polishing a focusing surface 1238 onto the input end 1239. In someembodiments, the focusing surface 1238 is coated with a dielectricanti-reflection coating. In some embodiments, a tube 1234 (e.g., a glassferrule tube, in some embodiments) is laser-welded (joint 1235) at oneend to the input end of hollow-core PC delivery fiber 1230, to seal outcontaminants from the pump block 1218 (see FIG. 12A). In someembodiments, lens-shaped fused cap 1239 is anti-reflection coated at itsinner and/or outer surfaces, in order to reduce detrimental reflections.In some embodiments, the length of the fused portion of cap 1239 issufficient to allow the lens portion of cap 1239 to focus an incidentbeam into the hollow core of delivery fiber 1230.

Fiber- or Rod-Based Optical Source Featuring a Large-Core,Rare-Earth-Doped Photonic-Crystal Device for Generation of High-PowerPulsed Radiation and Method

In some embodiments, the present invention provides an apparatus thatincludes a first photonic-crystal optical device that includes a firstwaveguide that has a diameter of about 40 microns or more and maintainsa single transverse mode. In some embodiments, the first waveguide has adiameter of more than 40 microns. In some embodiments, the first opticaldevice is a first photonic-crystal fiber and the first waveguide is afirst core of the first photonic-crystal device and is capable ofoperation with a peak power of about 1 MW or more. In some embodiments,the first core of the first photonic-crystal fiber is capable ofoperation with a peak power of more than 1 MW. In some embodiments, thefirst core of the first photonic-crystal device is capable of operationwith a near-diffraction-limited output beam having M²<1.2. In someembodiments, the first core of the first photonic-crystal device iscapable of operation with a near-diffraction-limited output beam havingM²<1.2 to generate linearly polarized pulses and a peak power of about100 kW or more.

Some embodiments further include one or more wavelength-conversionoptical media (e.g., non-linear frequency doublers, optical parametricoscillators and the like) operable to receive high-peak-power inputradiation having a first wavelength from the first waveguide of thefirst photonic-crystal device and to generate radiation having a peakpower of about 100 kW or more and of a shorter second wavelength throughwavelength conversion. In some embodiments, the second wavelengthradiation includes visible light having a wavelength between about 400nm and about 700 nm. In some embodiments, the second wavelengthradiation includes ultraviolet light having a wavelength of about 400 nmor shorter.

Some embodiments further include at least one wavelength-conversionoptical medium (e.g., non-linear frequency doublers, optical parametricoscillators and the like) operable to receive high-peak-power inputradiation having a first wavelength from the first waveguide of thefirst photonic-crystal device and to generate radiation having a peakpower of at least about 100 kW and of a different second wavelengththrough wavelength conversion. In some embodiments, thesecond-wavelength radiation is of a shorter wavelength than the firstwavelength. In some such embodiments, the radiation of the shortersecond wavelength is at least 200 kW. In some such embodiments, theradiation of the shorter second wavelength is at least 300 kW. In somesuch embodiments, the radiation of the shorter second wavelength is atleast 400 kW. In some such embodiments, the radiation of the shortersecond wavelength is at least 500 kW. In some such embodiments, theradiation of the shorter second wavelength is at least 600 kW. In somesuch embodiments, the radiation of the shorter second wavelength is atleast 700 kW. In some such embodiments, the radiation of the shortersecond wavelength is at least 800 kW. In some such embodiments, theradiation of the shorter second wavelength is at least 900 kW. In somesuch embodiments, the radiation of the shorter second wavelength is atleast 1000 kW. In some embodiments, the radiation of the shorter secondwavelength includes visible light having a wavelength between about 400nm and about 700 nm. In some embodiments, the second-wavelengthradiation includes ultraviolet light having a wavelength of about 400 nmor shorter. In other embodiments, the second-wavelength radiation is ofa longer wavelength than the first wavelength. In some such embodiments,the radiation of the longer second wavelength is at least 200 kW. Insome such embodiments, the radiation of the longer second wavelength isat least 300 kW. In some such embodiments, the radiation of the longersecond wavelength is at least 400 kW. In some such embodiments, theradiation of the longer second wavelength is at least 500 kW.

In some embodiments, the first optical device is a firstphotonic-crystal fiber and the first waveguide is a first core of thefirst photonic-crystal device that supports a mode area having adiameter of about 50 microns or larger and a cladding having a diameterof about 1,000 microns or larger. In some embodiments, the first opticaldevice is a first photonic-crystal fiber and the first waveguide is afirst core of the first photonic-crystal device, and the apparatusfurther includes a first optical isolator, a first narrow-bandwidthfilter, and a master-oscillator subsystem operable to generate anarrow-linewidth seed-laser signal operably coupled to the first core ofthe first photonic-crystal fiber through the first optical isolator andthe first narrow-bandwidth filter. In some embodiments, themaster-oscillator subsystem further includes a second optical isolator,a second narrow-bandwidth filter, a second photonic-crystal-fiberoptical amplifier having a core, and a master-oscillator seed laseroperable to generate a seed-laser signal operably coupled to the core ofthe second photonic-crystal fiber through the second optical isolatorand the second narrow-bandwidth filter, wherein the secondphotonic-crystal fiber outputs the narrow-linewidth seed-laser signal.

In some embodiments, the first waveguide of the first photonic-crystaldevice is surrounded by a plurality of longitudinal holes that define atransverse extent of the first waveguide, and wherein the holes areclosed for a first length at a first end of the first photonic-crystalfiber to form an endcap. In some embodiments, the first optical deviceis a first photonic-crystal fiber and the first waveguide is a firstcore of the first photonic-crystal device, and wherein the holes of thefirst core of the first photonic-crystal fiber have been melted shut forthe first length. In some embodiments, the holes of the first waveguideof the first photonic-crystal device have been filled with anindex-matching material for the first length. In some embodiments, theendcap of the first photonic-crystal fiber is formed to a diametersmaller than a diameter of the first fiber away from the endcap, and afacet is formed at an end of the endcap of the first fiber. Someembodiments further include a third photonic-crystal fiber having anendcap that is formed to a diameter smaller than a diameter of the thirdfiber away from the endcap, and having a facet formed at an end of theendcap of the third fiber, wherein the endcap of the first fiber isplaced side-by-side to the endcap of the third fiber such that the endfacet of the first fiber and the end facet of the second fiber areplaced at a center-to-center distance smaller than the diameter of thefirst fiber away from the endcap of the first fiber.

In some embodiments, the first photonic-crystal device further includesa second waveguide that has a diameter of about 40 microns or more andmaintains a single transverse mode. In some embodiments, the firstoptical device is a first photonic-crystal fiber and the first waveguideis a first core of the first photonic-crystal device, and wherein thefirst photonic-crystal fiber further includes a plurality of other coresthat each have a diameter of about 40 microns or more and that eachmaintain a single transverse mode. In some embodiments, the firstoptical device is a first photonic-crystal fiber and the first waveguideis a first core of the first photonic-crystal device, and wherein thefirst photonic-crystal fiber further includes a plurality of other coresarranged side-by-side generally along a straight line transverse to alength of the first fiber, and wherein the cores each have a diameter ofabout 40 microns or more and each maintain a single transverse mode,wherein the fiber includes an inner pump cladding surrounding the coresin order to provide pump light into the cores over a length of thefiber, and an outer cladding that contains the pump light.

In some embodiments, the invention provides a method that includesproviding a first photonic-crystal device that includes a firstwaveguide having a diameter of about 40 microns or more, and opticallyamplifying light in the first waveguide in a single transverse mode. Insome embodiments, the first waveguide has a diameter of more than 40microns. In some embodiments, the amplifying of the light in the firstwaveguide generates a peak power of about 1 MW or more. In someembodiments, the amplifying of the light in the first waveguidegenerates a peak power of more than 1 MW. In some embodiments, theamplifying of the light in the first waveguide generates anear-diffraction-limited output beam having M²<1.2. In some embodiments,the amplifying of the light in the first waveguide generates anear-diffraction-limited output beam having (M²<1.2) linearly polarizedpulses having a peak power of about 100 kW or more.

Some embodiments of the method further include convertinghigh-peak-power light having a first wavelength from the first waveguideof the first photonic-crystal device to generate light having a peakpower of about 100 kW or more of a shorter second wavelength throughnon-linear wavelength conversion. In some embodiments, thesecond-wavelength radiation includes visible light having a wavelengthbetween about 400 nm and about 700 nm. In some embodiments, thesecond-wavelength radiation includes ultraviolet light having awavelength of about 400 nm or shorter. In some embodiments, the firstcore of the first photonic-crystal device supports a mode area having adiameter of about 50 microns or larger and has a cladding having adiameter larger than the core. In some embodiments, the firstphotonic-crystal device is a rod having an outer diameter of about 1,000microns (1 mm) or larger. In some embodiments, the first optical deviceis a first photonic-crystal fiber and the first waveguide is a firstcore of the first photonic-crystal device, and the method furtherincludes generating a narrow-linewidth seed-laser signal, opticallyisolating the narrow-linewidth seed-laser signal, narrow-bandwidthfiltering the narrow-linewidth seed-laser signal, and amplifying theisolated filtered narrow-linewidth seed-laser signal using the firstwaveguide of the first photonic-crystal device. In some embodiments, thegenerating of the narrow-linewidth seed-laser signal further includesgenerating an original seed-laser signal, optically isolating theoriginal seed-laser signal, narrow-bandwidth filtering the originalseed-laser signal, providing a second photonic-crystal fiber opticalamplifier having a core, and amplifying the isolated filtered originalseed-laser signal using the core of the second photonic-crystal fiber,wherein the second photonic-crystal fiber outputs the narrow-linewidthseed-laser signal. In some embodiments, the first waveguide of theprovided first photonic-crystal device is surrounded by a plurality oflongitudinal holes that define a transverse extent of the firstwaveguide, and the method further includes closing the holes for a firstlength at a first end of the first photonic-crystal fiber to form anendcap. In some embodiments, the closing of the holes of the firstwaveguide of the first photonic-crystal fiber includes melting the holesshut for the first length. In some embodiments, the closing of the holesof the first waveguide of the first photonic-crystal fiber includesfilling the holes with an index-matching material for the first length.In some embodiments, the first optical device is a firstphotonic-crystal fiber and the first waveguide is a first core of thefirst photonic-crystal device, and the method further includes formingthe endcap of the first photonic-crystal fiber to a diameter smallerthan a diameter of the first fiber away from the endcap, and forming afacet at an end of the endcap of the first fiber. Some embodimentsfurther include providing a third photonic-crystal fiber, forming anendcap on an end of the third fiber to a diameter smaller than adiameter of the third fiber away from the endcap, forming a facet at anend of the endcap of the third fiber, and placing the endcap of thefirst fiber side-by-side to the endcap of the third fiber such that theend facet of the first fiber and the end facet of the second fiber areplaced at a center-to-center distance smaller than the diameter of thefirst fiber away from the endcap of the first fiber.

In some embodiments, the first photonic-crystal device further includesa second waveguide that has a diameter of about 40 microns or more, andthe method further includes optically amplifying light in the secondwaveguide with a single transverse mode. In some embodiments, the firstphotonic-crystal fiber further includes a plurality of other waveguidesthat each have a diameter of about 40 microns or more, and the methodfurther includes maintaining a single transverse mode in each of theplurality of waveguides. In some embodiments, the first optical deviceis a first photonic-crystal fiber and the first waveguide is a firstcore of the first photonic-crystal fiber, wherein the firstphotonic-crystal fiber further includes a plurality of other coresarranged side-by-side generally along a straight line transverse to alength of the first fiber, wherein the fiber includes an inner pumpcladding surrounding the plurality of cores and an outer claddingsurrounding the inner cladding, and wherein the cores each have adiameter of about 40 microns or more, and the method further includesmaintaining a single transverse mode in each of the plurality of cores,and providing pump light into the inner pump cladding surrounding thecores in order to provide pump light into the cores over a length of thefiber, and containing the pump light inside an outer-extent radius ofthe inner cladding.

In some embodiments, the present invention provides an apparatus thatincludes a photonic-crystal optical device having a waveguide that has adiameter of about 40 microns or more and is doped with rare-earth ions(or other rare-earth species) capable of optical amplification andmaintains a single transverse mode and is capable of operation with apeak power of about 1 MW or more when used as a pulsed optical amplifieror laser. In some embodiments, the waveguide has a diameter of at least40 microns. In some embodiments, the waveguide has a diameter of morethan 40 microns. In some embodiments, the waveguide has a diameter of atleast 45 microns. In some embodiments, the waveguide has a diameter ofat least 50 microns. In some embodiments, the waveguide has a diameterof more than 50 microns. In some embodiments, the waveguide has adiameter of at least 55 microns. In some embodiments, the waveguide hasa diameter of at least 60 microns. In some embodiments, the waveguidehas a diameter of at least 65 microns. In some embodiments, thewaveguide has a diameter of at least 70 microns. In some embodiments,the waveguide has a diameter of at least 85 microns. In someembodiments, the waveguide has a diameter of at least 80 microns. Insome embodiments, the waveguide has a diameter of at least 85 microns.In some embodiments, the waveguide has a diameter of at least 90microns. In some embodiments, the waveguide has a diameter of at least95 microns. In some embodiments, the waveguide has a diameter of atleast 100 microns. In some embodiments, the waveguide has a diameter ofmore than 100 microns. In some embodiments, the waveguide has a diameterof at least 105 microns. In some embodiments, the waveguide has adiameter of at least 110 microns. In some embodiments, the waveguide hasa diameter of at least 120 microns. In some embodiments, the waveguidehas a diameter of at least 130 microns. In some embodiments, thewaveguide has a diameter of at least 140 microns. In some embodiments,the waveguide has a diameter of at least 150 microns. In someembodiments, the waveguide has a diameter of at least 200 microns.

In some embodiments, the optical device includes a photonic-crystalfiber and the waveguide is a core of the photonic-crystal fiber. In someembodiments, the fiber also includes an inner cladding surrounding thecore in order to provide pump light into the cores over a length of thefiber.

In some such embodiments, the core of the photonic-crystal fiber iscapable of operation with a peak power of at least 200 kW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of at least 300 kW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of at least 400 kW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of at least 500 kW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of at least 600 kW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of at least 700 kW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of at least 800 kW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of at least 900 kW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of at least 1000 kW.

In some such embodiments, the core of the photonic-crystal fiber iscapable of operation with a peak power of at least 1100 kW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of at least 1200 kW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of at least 1300 kW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of at least 1400 kW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of at least 1500 kW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of at least 1600 kW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of at least 1700 kW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of at least 1800 kW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of at least 1900 kW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of at least 2000 kW.

In some such embodiments, the core of the photonic-crystal fiber iscapable of operation with a peak power of at least 2100 kW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of at least 2200 kW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of at least 2300 kW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of at least 2400 kW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of at least 2500 kW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of at least 2600 kW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of at least 2700 kW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of at least 2800 kW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of at least 2900 kW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of at least 3000 kW.

In some such embodiments, the core of the photonic-crystal fiber iscapable of operation with a peak power of at least 3100 kW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of at least 3200 kW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of at least 3300 kW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of at least 3400 kW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of at least 3500 kW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of at least 3600 kW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of at least 3700 kW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of at least 3800 kW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of at least 3900 kW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of at least 4000 kW.

In some such embodiments, the core of the photonic-crystal fiber iscapable of operation with a peak power of more than 0.5 MW. In some suchembodiments, the core of the photonic-crystal fiber is capable ofoperation with a peak power of more than 1 MW. In some such embodiments,the core of the photonic-crystal fiber is capable of operation with apeak power of more than 2 MW. In some such embodiments, the core of thephotonic-crystal fiber is capable of operation with a peak power of morethan 3 MW. In some such embodiments, the core of the photonic-crystalfiber is capable of operation with a peak power of more than 4 MW. Insome of these embodiments, the core of the photonic-crystal device iscapable of operation at these powers with a near-diffraction-limitedoutput beam having M²<1.5. In some embodiments, the output beam includeslinearly polarized pulses having a degree of polarization of at least 15dB (wherein the degree of polarization is defined as ten times the log(base 10) of the ratio of optical power along the polarization axis tothe optical power along the orthogonal axis). In some such embodiments,the degree of polarization is at least 16 dB. In some embodiments, theoutput beam includes linearly polarized pulses having a degree ofpolarization of at least 17 dB. In some such embodiments, the degree ofpolarization is at least 18 dB. In some such embodiments, the degree ofpolarization is at least 19 dB. In some embodiments, the output beamincludes linearly polarized pulses having a degree of polarization of atleast 20 dB.

In some embodiments, the core of the photonic-crystal device is capableof operation with a near-diffraction-limited output beam having M²<1.5.

In some embodiments, the core of the photonic-crystal device is capableof operation with a near-diffraction-limited output beam having M²<1.5to generate linearly polarized pulses and a peak power of about 100 kWor more. In some such embodiments, the core of the photonic-crystaldevice is capable of peak power of about 200 kW or more. In some suchembodiments, the core of the photonic-crystal device is capable of peakpower of about 300 kW or more. In some such embodiments, the core of thephotonic-crystal device is capable of peak power of about 500 kW ormore.

Some embodiments further include one or more wavelength-conversionoptical media operable to receive high-peak-power input radiation havinga first wavelength from the first waveguide of the firstphotonic-crystal device and to generate an output beam radiation havinga peak power of at least about 100 kW and of a shorter second wavelengththrough wavelength conversion.

Some embodiments further include one or more wavelength-conversionoptical media operable to receive high-peak-power input radiation havinga first wavelength from the first waveguide of the firstphotonic-crystal device and to generate radiation having a peak power ofabout 100 kW or more and of a second wavelength (different than thefirst wavelength) through wavelength conversion. In some embodiments,the output beam radiation of the second wavelength obtained throughwavelength conversion exhibits peak power of 300 kW or more. In someembodiments, the output beam radiation of the second wavelength obtainedthrough wavelength conversion exhibits peak power of 400 kW or more. Insome embodiments, the output beam radiation of the second wavelengthobtained through wavelength conversion exhibits peak power of 500 kW ormore. In some embodiments, the output beam radiation of the secondwavelength obtained through wavelength conversion includes visible lighthaving a wavelength between about 400 nm and about 700 nm. In someembodiments, the output beam radiation of the second wavelength obtainedthrough wavelength conversion includes ultraviolet light having awavelength of about 400 nm or shorter. In some embodiments, the outputbeam radiation of the second wavelength includes a wavelength longerthan the first wavelength.

In some embodiments, the optical device includes a photonic-crystalfiber and the waveguide is the core of the photonic-crystal fiber thatsupports a fundamental mode field having a diameter of at least about 50microns and has a pump cladding having a diameter larger than that ofthe core. In some such embodiments, the fundamental mode field has adiameter of at least 60 microns. In some such embodiments, thefundamental mode field as a diameter of at least 70 microns. In somesuch embodiments, the fundamental mode field as a diameter of at least80 microns. In some such embodiments, the fundamental mode field as adiameter of at least 90 microns. In some such embodiments, thefundamental mode field as a diameter of at least 100 microns.

Another aspect of the present invention provides an apparatus thatincludes a photonic-crystal optical amplifier having a waveguide thathas a diameter of about 40 microns or more, that is doped with at leastone rare-earth species capable of optical amplification, that maintainsa single transverse mode, and that is capable of operation with a peakpower of at least about 1 MW when used as a pulsed optical amplifier orlaser, wherein the optical device includes a photonic-crystal fiber andthe waveguide is a first core of the photonic-crystal device, theapparatus further including a first optical isolator; a firstwavelength-sensitive optical filter, and a master-oscillator subsystemoperable to generate a narrow-linewidth, single-frequency seed-lasersignal coupled to the first core of the first photonic-crystal fiberthrough the first optical isolator and the first wavelength-sensitiveoptical filter. In some such embodiments, the first wavelength-sensitiveoptical filter includes a narrow-bandwidth band-pass filter. In someembodiments, the first wavelength-sensitive optical filter includes along-pass optical filter. In some embodiments, the firstwavelength-sensitive optical filter includes a short-pass opticalfilter.

Multi-Stage Optical Amplifier Having Photonic-Crystal Waveguides forGeneration of High-Power Pulsed Radiation and Associated Method

In some embodiments, the present invention provides an apparatusincluding an optical amplifier having a segmented photonic-crystal fiberthat includes a first amplifying segment having a rare-earth-dopedphotonic-crystal core and a second amplifying segment having arare-earth-doped photonic-crystal core, a master oscillator operable togenerate a seed-laser signal, and a first optical connector subassemblyoperatively coupled between the first segment and the second segment. Insome embodiments, the master-oscillator subsystem includes a firstoptical isolator and a first narrow-bandwidth band-pass optical filter,and is operable to generate a narrow-linewidth, single-frequencyseed-laser signal coupled to the core of the first segment through thefirst optical isolator and the first narrow-bandwidth band-pass opticalfilter. In some embodiments, the master-oscillator subsystem includes afirst optical isolator and a first long-pass optical filter, and isoperable to generate a narrow-linewidth, single-frequency seed-lasersignal coupled to the core of the first segment through the firstoptical isolator and the first long-pass optical filter.

In some embodiments, the first optical connector subassembly includes apump block, wherein the pump block includes a pump-light injection port,a dichroic device that operates to convey signal wavelengths from thefirst segment towards the second segment and to convey pump wavelengthsfrom the pump-light injection port towards the first segment, and anoptical filter through which the signal passes and that blocks at leastsome wavelengths other than the signal's primary wavelength.

In some embodiments, a peak power of the signal's optical pulses emittedby the second segment is at least 1 MW and a ratio between optical powerin the signal's optical pulses and optical power in a continuous-wavebackground emitted by the second segment is at least 15 dB. In someembodiments, a ratio between optical power in the signal's opticalpulses and optical power in a continuous-wave background emitted by thesecond segment is at least 16 dB. In some embodiments, a ratio betweenoptical power in the signal's optical pulses and optical power in acontinuous-wave background emitted by the second segment is at least 17dB. In some embodiments, a ratio between optical power in the signal'soptical pulses and optical power in a continuous-wave background emittedby the second segment is at least 18 dB. In some embodiments, a ratiobetween optical power in the signal's optical pulses and optical powerin a continuous-wave background emitted by the second segment is atleast 19 dB. In some embodiments, a ratio between optical power in thesignal's optical pulses and optical power in a continuous-wavebackground emitted by the second segment is at least 20 dB.

In some embodiments, a pulse energy of the optical pulses emitted by thesecond segment of photonic-crystal fiber is at least 1 mJ and the pulsehas a duration of no more than 5 ns. In some embodiments, a pulse energyof the optical pulses emitted by the second segment of photonic-crystalfiber is at least 1 mJ and the pulse has a duration of no more than 4ns. In some embodiments, a pulse energy of the optical pulses emitted bythe second segment of photonic-crystal fiber is at least 1 mJ and thepulse has a duration of no more than 3 ns. In some embodiments, a pulseenergy of the optical pulses emitted by the second segment ofphotonic-crystal fiber is at least 2 mJ and the pulse has a duration ofno more than 5 ns. In some embodiments, a pulse energy of the opticalpulses emitted by the second segment of photonic-crystal fiber is atleast 2 mJ and the pulse has a duration of no more than 4 ns. In someembodiments, a pulse energy of the optical pulses emitted by the secondsegment of photonic-crystal fiber is at least 2 mJ and the pulse has aduration of no more than 3 ns. In some embodiments, a pulse energy ofthe optical pulses emitted by the second segment of photonic-crystalfiber is at least 3 mJ and the pulse has a duration of no more than 3ns. In some embodiments, a pulse energy of the optical pulses emitted bythe second segment of photonic-crystal fiber is at least 4 mJ and thepulse has a duration of no more than 5 ns. In some embodiments, a pulseenergy of the optical pulses emitted by the second segment ofphotonic-crystal fiber is at least 4 mJ and the pulse has a duration ofno more than 4 ns.

In some embodiments, a pulse energy of the optical pulses emitted by thesecond segment of photonic-crystal fiber is at least 4 mJ and the pulsehas a duration of no more than 3 ns.

In some embodiments, the seed-laser signal from the master oscillator iscoupled into the photonic-crystal core of the first segment andamplified therein and an output of the first segment is transmittedthrough the first optical connector subassembly and then coupled intothe photonic-crystal core of the second segment and further amplifiedtherein and the amplified signal is emitted through an output end of thesecond segment.

In some embodiments, the plurality of segments arranged in series isoperated as a composite, single optical amplifier for the seed-lasersignal emitted from the master oscillator. In some such embodiments, thefirst optical connector subassembly includes a short multimode piece offiber that is fused to an output end of the first segment and to aninput end of the second segment. Some embodiments further include athird segment having a photonic-crystal core and a second opticalconnector subassembly operable connected between the master oscillatorand the first segment.

In some embodiments, the seed-laser signal emitted by the masteroscillator is optically amplified in a plurality of rare-earth-dopedphotonic crystal fiber segments arranged in series and eachrare-earth-doped photonic-crystal fiber segment provides a fraction ofthe overall optical gain experienced by the seed-laser signal.

In some embodiments, each optical subassembly positioned betweensuccessive segments of rare-earth-doped photonic-crystal fiber includesat least one collimating lens, at least one optical filter thatseparates light of the wavelength of the signal pulses from light of thewavelength of the pump laser, at least one narrow-band optical band-passfilter and at least one focusing lens. In some such embodiments, atleast one optical subassembly further includes at least one opticalisolator.

In some embodiments, at least one optical component included in at leastone optical subassembly positioned between successive segments ofrare-earth-doped photonic-crystal fiber is laser-welded to an enclosure.

In some embodiments, the narrow-band optical band-pass filter includedin at least one optical subassembly is operated also as an opticalisolator that blocks light at wavelengths other than the signal pulsesthat counter-propagate with respect to the signal pulses.

In some embodiments, at least one optical component included in at leastone optical subassembly positioned between successive pieces ofrare-earth-doped photonic-crystal fiber is soldered to an enclosure.

In some embodiments, an end of at least one segment of photonic-crystalfiber is attached to the first optical subassembly using connectors thatdo not contain any epoxy or other organic material. In some suchembodiments, at least one connector includes a hollow ferrule made of anoptically clear material, laser-welded to an outer surface of an end ofa segment of photonic-crystal fiber. In some such embodiments, thehollow ferrule is attached to the photonic-crystal fiber end bythermally induced shrinkage.

In some embodiments, the ends of at least one segment ofphotonic-crystal fiber are laser-welded to an enclosure.

In some embodiments, the ends of at least one segment ofphotonic-crystal fiber are soldered to an enclosure.

In some embodiments, the optical beam emitted by at least one segment islinearly polarized with a degree of polarization being at least 15 dB.In some embodiments, the optical beam emitted by at least one segment islinearly polarized with a degree of polarization being at least 16 dB.In some embodiments, the optical beam emitted by at least one segment islinearly polarized with a degree of polarization being at least 17 dB.In some embodiments, the optical beam emitted by at least one segment islinearly polarized with a degree of polarization being at least 18 dB.In some embodiments, the optical beam emitted by at least one segment islinearly polarized with a degree of polarization being at least 19 dB.In some embodiments, the optical beam emitted by at least one segment islinearly polarized with a degree of polarization being at least 20 dB.

Some embodiments further include at least one wavelength-conversionoptical medium, operable to receive as input a high-peak-poweroptical-signal beam from the second amplifying segment having a firstwavelength, and to generate through wavelength conversion an outputoptical beam having a second wavelength and a peak power of at leastabout 100 kW.

In some embodiments, the second-wavelength optical beam has a wavelengthshorter than that of the first-wavelength beam. In some suchembodiments, the peak power of the second-wavelength optical beam is 200kW or more. In some such embodiments, the peak power of thesecond-wavelength optical beam is 300 kW or more. In some suchembodiments, the peak power of the second-wavelength optical beam is 400kW or more. In some such embodiments, the peak power of thesecond-wavelength optical beam is 500 kW or more. In some suchembodiments, the peak power of the second-wavelength optical beam is 600kW or more. In some such embodiments, the peak power of thesecond-wavelength optical beam is 700 kW or more. In some suchembodiments, the peak power of the second-wavelength optical beam is 800kW or more. In some such embodiments, the peak power of thesecond-wavelength optical beam is 900 kW or more. In some suchembodiments, the peak power of the second-wavelength optical beam is1000 kW or more.

In some embodiments, the second-wavelength optical beam has a wavelengthlonger than the first-wavelength beam. In some such embodiments, thepeak power of the longer-wavelength optical beam is 200 kW or more. Insome such embodiments, the peak power of the longer-wavelength opticalbeam is 300 kW or more. In some such embodiments, the peak power of thelonger-wavelength optical beam is 400 kW or more. In some suchembodiments, the peak power of the longer-wavelength optical beam is 500kW or more.

Photonic-Crystal Rod Amplifiers for High-Power Pulsed Optical Radiationand associated method

In some embodiments, the present invention provides an apparatus thatincludes a first photonic-crystal rod (PCR), having rare-earth-dopedcore with a diameter of at least 40 microns and an external diameter ofat least 1 mm such that the rod is therefore thick enough tosubstantially and readily hold its shape when released. In some suchembodiments, the core of the PCR has a diameter of at least 50 microns.In some such embodiments, the core of the PCR has a diameter of at least60 microns. In some such embodiments, the core of the PCR has a diameterof at least 70 microns. In some such embodiments, the core of the PCRhas a diameter of at least 80 microns. In some such embodiments, thecore of the PCR has a diameter of at least 90 microns. In some suchembodiments, the core of the PCR has a diameter of at least 100 microns.In some such embodiments, the core of the PCR has a diameter of at least110 microns. In some such embodiments, the core of the PCR has adiameter of at least 120 microns. In some such embodiments, the core ofthe PCR has a diameter of at least 130 microns. In some suchembodiments, the core of the PCR has a diameter of at least 140 microns.In some such embodiments, the core of the PCR has a diameter of at least150 microns. In some embodiments, the apparatus is operable to generatea peak signal power of at least 500 kW.

In some embodiments, the optical beam emitted by the PCR is linearlypolarized with a degree of polarization being at least 15 dB. In someembodiments, the optical beam emitted by at least one segment islinearly polarized with a degree of polarization being at least 16 dB.In some embodiments, the optical beam emitted by the PCR is linearlypolarized with a degree of polarization being at least 17 dB. In someembodiments, the optical beam emitted by at least one segment islinearly polarized with a degree of polarization being at least 18 dB.In some embodiments, the optical beam emitted by at least one segment islinearly polarized with a degree of polarization being at least 19 dB.In some embodiments, the optical beam emitted by the PCR is linearlypolarized with a degree of polarization being at least 20 dB.

Some embodiments further include at least one wavelength-conversionoptical medium, operable to receive as input a high-peak-poweroptical-signal beam from the PCR having a first wavelength, and togenerate through wavelength conversion an output optical beam having asecond wavelength and a peak power of at least about 100 kW.

In some embodiments, the second-wavelength optical beam has a wavelengthshorter than that of the first-wavelength beam. In some suchembodiments, the peak power of the shorter second-wavelength opticalbeam is 200 kW or more. In some such embodiments, the peak power of theshorter second-wavelength optical beam is 300 kW or more. In some suchembodiments, the peak power of the shorter second-wavelength opticalbeam is 400 kW or more. In some such embodiments, the peak power of theshorter second-wavelength optical beam is 500 kW or more. In some suchembodiments, the peak power of the shorter second-wavelength opticalbeam is 600 kW or more. In some such embodiments, the peak power of theshorter second-wavelength optical beam is 700 kW or more. In some suchembodiments, the peak power of the shorter second-wavelength opticalbeam is 800 kW or more. In some such embodiments, the peak power of theshorter second-wavelength optical beam is 900 kW or more. In some suchembodiments, the peak power of the shorter second-wavelength opticalbeam is 1000 kW or more.

In some embodiments, the second-wavelength optical beam has a wavelengthlonger than the first-wavelength beam. In some such embodiments, thepeak power of the longer second-wavelength optical beam is 200 kW ormore. In some such embodiments, the peak power of the longersecond-wavelength optical beam is 300 kW or more. In some suchembodiments, the peak power of the longer second-wavelength optical beamis 400 kW or more. In some such embodiments, the peak power of thelonger second-wavelength optical beam is 500 kW or more.

Multi-Segment Photonic-Crystal-Rod Waveguides for Amplification ofHigh-Power Pulsed Optical Radiation and Associated Method

In some embodiments, the present invention provides an apparatus thatincludes a first segmented photonic-crystal rod (PCR), havingrare-earth-doped core with a diameter of at least 40 microns and anexternal diameter of at least 1 mm such that the rod is therefore thickenough to substantially and readily hold its shape when released, andwherein the photonic-crystal rod is configured as at least two segmentsthat are serially encountered along a signal's optical path. In someembodiments, each two consecutive PCR segments in the series are joinedusing a piece of bridge optical fiber having an outer diameter differentfrom that of the PCR segments and its ends spliced to the ends of thePCR segments so as to form a chain of alternating PCR segments andbridge-fiber segments, wherein the chain is operable as a continuousoptical waveguide. In some such embodiments, each bridge fiber is fusedat its ends to its two PCR segments. In some such embodiments, eachbridge fiber is laser welded to its two PCR segments. In some suchembodiments, the laser welding acts to seal ends of the PCR's core'sholes.

In some embodiments, at least one of the PCR segments is arranged at anon-zero angle to (and non-parallel with respect to) a preceding PCRsegment in the chain. In some embodiments, at least one of the PCRsegments is arranged in parallel with, but non-co-linear with, apreceding PCR segment in the chain.

In some embodiments, at least one piece of bridge fiber in the chain hasan un-doped core having a diameter of at least 50 microns and a corenumerical aperture sufficiently high to withstand bending with lowoptical loss for the fundamental transverse mode of the core.

In some embodiments, at least one piece of bridge fiber in the chainfeatures a core surrounded by a concentric cladding that transports pumplight from one PCR segment to the next. In some embodiments, the bridgefiber is a double-clad fiber.

In some embodiments, at least one piece of bridge fiber in the chain hasa rare-earth-doped core. In some embodiments, the doped core of thebridge fiber provides additional amplification to the signal.

In some embodiments, at least one piece of bridge fiber in the chain hasa hollow core.

In some embodiments, at least one pair of the photonic-crystal-rodsegments in the series is interspaced by a free-space gap and isoperated such that the optical beam exiting one segment is coupled intothe successive segment using a plurality of optical components. In somesuch embodiments, the plurality of optical components used to couple theoptical beam from a PCR segment into the successive PCR segment includeat least one lens and at least one optical prism. In some embodiments,the PCR segments are arranged so that at least one PCR segment forms anon-zero angle with respect to a preceding one.

In some embodiments, at least one of the PCR segments has one of its endfacets shaped to form a lens that collimates the optical beampropagating outward from the core of the segment. In some embodiments,at least one of the PCR segments has one of its end facets shaped toform a lens that focuses into the PCR segment's core an optical beamthat is coupled into the segment from outside the segment.

In some embodiments, the apparatus includes at least twophotonic-crystal-rod (PCR) segments arranged side by side such thattheir cores run parallel to one another and the optical beam exiting afirst segment is coupled into a second segment without the aid ofexternal optical components (for example, as shown in FIG. 11G, wherethe sides of the segments are laser-welded to one another to prevent airgaps, at least in the optical path), such that the optical beamtraverses the first segment from left to right, the second segment fromright to left. In some embodiments, the apparatus includes at leastthree photonic-crystal-rod (PCR) segments arranged side by side suchthat their cores run parallel to one another and the optical beamexiting one or more segments is coupled into the successive segmentwithout the aid of external optical components, but rather by reflectiveoutput facets and input facets that are shaped to direct an optical beamexiting one segment into the core of an adjacent segment, such that theoptical beam traverses one segment from left to right, the successivesegment from right to left, and the successive segment from right toleft, in a zigzag fashion. In some such embodiments, at least one of thePCR segments has its input-end facet and output-end facet formed intocurved reflective surfaces such that the input facet reflects an inputoptical beam at an angle and focuses it into the core of the PCR segmentand the output facet reflects at an angle the optical beam exiting thecore of the PCR segment and collimates this beam. In some suchembodiments, the facets of the at least one PCR segment are coated forhigh reflectivity at the wavelengths of the optical beam propagating inthe segment. In some embodiments, the reflective coating is amulti-layered dielectric coating.

In some embodiments, at least two of the PCR segments have an externalsurface laser-welded to that of a neighboring PCR segment such that thelaser-welded seam runs parallel to their cores and no free-space gap isencountered by the optical signal beam between exiting one segment andentering the successive segment that is laser-welded to it, the path ofthe optical beam being entirely confined within the bodies of theadjacent segments so as to avoid interfacial optical loss.

In some embodiments, at least one of the PCR segments outputs a linearlypolarized optical beam having degree of polarization of at least 15 dB.In some such embodiments, the degree of polarization is at least 16 dB.In some such embodiments, the degree of polarization is at least 17 dB.In some such embodiments, the degree of polarization is at least 18 dB.In some such embodiments, the degree of polarization is at least 19 dB.In some such embodiments, the degree of polarization is at least 20 dB.

Some embodiments further include at least one wavelength-conversionoptical medium, operable to receive as input a high-peak-poweroptical-signal beam from the segmented PCR having a first wavelength,and to generate through wavelength conversion an output optical beamhaving a second wavelength and a peak power of at least about 100 kW.

In some embodiments, the second-wavelength optical beam has a wavelengthshorter than that of the first-wavelength beam. In some suchembodiments, the peak power of the second-wavelength optical beam is 200kW or more. In some such embodiments, the peak power of thesecond-wavelength optical beam is 300 kW or more. In some suchembodiments, the peak power of the second-wavelength optical beam is 400kW or more. In some such embodiments, the peak power of thesecond-wavelength optical beam is 500 kW or more. In some suchembodiments, the peak power of the second-wavelength optical beam is 600kW or more. In some such embodiments, the peak power of thesecond-wavelength optical beam is 700 kW or more. In some suchembodiments, the peak power of the second-wavelength optical beam is 800kW or more. In some such embodiments, the peak power of thesecond-wavelength optical beam is 900 kW or more. In some suchembodiments, the peak power of the second-wavelength optical beam is1000 kW or more.

In some embodiments, the second-wavelength optical beam has a wavelengthlonger than the first-wavelength beam. In some such embodiments, thepeak power of the longer-wavelength optical beam is 200 kW or more. Insome such embodiments, the peak power of the longer-wavelength opticalbeam is 300 kW or more. In some such embodiments, the peak power of thelonger-wavelength optical beam is 400 kW or more. In some suchembodiments, the peak power of the longer-wavelength optical beam is 500kW or more.

Multi-Stage Optical Amplifier Having Photonic-Crystal-Rod Waveguides andNormal Optical Fiber and Associated Method

In some embodiments, the present invention provides an apparatusincluding an optical amplifier having a photonic-crystal rod having arare-earth-doped photonic-crystal core having a diameter of at least 40microns and a cladding having an outer diameter of at least 1 mm, amaster oscillator operable to generate a seed-laser signal, and a firstoptical component having a solid-body fiber optically coupled to thephotonic-crystal rod, wherein the solid-body fiber does not have aphotonic-crystal structure in the optical signal path.

In some embodiments, the master-oscillator subsystem includes a firstoptical isolator and a first narrow-bandwidth band-pass optical filter,and is operable to generate a narrow-linewidth, single-frequencyseed-laser signal coupled to the core of the first segment through thefirst optical isolator and the first narrow-bandwidth band-pass opticalfilter. In some embodiments, the master-oscillator subsystem includes afirst optical isolator and a first long-pass optical filter, and isoperable to generate a narrow-linewidth, single-frequency seed-lasersignal coupled to the core of the first segment through the firstoptical isolator and the first long-pass optical filter.

In some embodiments of the apparatus, the core of the photonic-crystalrod (PCR) has a diameter of at least 50 microns. In some suchembodiments, the core of the PCR has a diameter of at least 60 microns.In some such embodiments, the core of the PCR has a diameter of at least70 microns. In some such embodiments, the core of the PCR has a diameterof at least 80 microns. In some such embodiments, the core of the PCRhas a diameter of at least 90 microns. In some such embodiments, thecore of the PCR has a diameter of at least 100 microns. In some suchembodiments, the core of the PCR has a diameter of at least 110 microns.In some such embodiments, the core of the PCR has a diameter of at least120 microns. In some such embodiments, the core of the PCR has adiameter of at least 130 microns. In some such embodiments, the core ofthe PCR has a diameter of at least 140 microns. In some suchembodiments, the core of the PCR has a diameter of at least 150 microns.In some embodiments, the apparatus is operable to generate a peak signalpower of at least 500 kW.

In some embodiments, the photonic-crystal rod (PCR) is separated into aplurality of PCR segments. In some such embodiments, the solid-bodyfiber is fused to respective ends of two of the PCR segments to form anoptical path between their respective cores.

In some embodiments, the apparatus has an output beam that includeslinearly polarized pulses having a degree of polarization of at least 15dB (wherein the degree of polarization is defined as ten times the log(base 10) of the ratio of optical power along the polarization axis tothe optical power along the orthogonal axis). In some such embodiments,the degree of polarization is at least 16 dB. In some embodiments, theoutput beam includes linearly polarized pulses having a degree ofpolarization of at least 17 dB. In some such embodiments, the degree ofpolarization is at least 18 dB. In some such embodiments, the degree ofpolarization is at least 19 dB. In some embodiments, the output beamincludes linearly polarized pulses having a degree of polarization of atleast 20 dB.

Some embodiments further include at least one wavelength-conversionoptical medium (e.g., non-linear frequency doublers, optical parametricoscillators and the like) operable to receive high-peak-power inputradiation having a first wavelength from the first waveguide of thefirst photonic-crystal device and to generate radiation having a peakpower of at least about 100 kW and of a different second wavelengththrough wavelength conversion. In some embodiments, thesecond-wavelength radiation is of a shorter wavelength than the firstwavelength. In some such embodiments, the radiation of the shortersecond wavelength is at least 200 kW. In some such embodiments, theradiation of the shorter second wavelength is at least 300 kW. In somesuch embodiments, the radiation of the shorter second wavelength is atleast 400 kW. In some such embodiments, the radiation of the shortersecond wavelength is at least 500 kW. In some such embodiments, theradiation of the shorter second wavelength is at least 600 kW. In somesuch embodiments, the radiation of the shorter second wavelength is atleast 700 kW. In some such embodiments, the radiation of the shortersecond wavelength is at least 800 kW. In some such embodiments, theradiation of the shorter second wavelength is at least 900 kW. In somesuch embodiments, the radiation of the shorter second wavelength is atleast 1000 kW. In some embodiments, the radiation of the shorter secondwavelength includes visible light having a wavelength between about 400nm and about 700 nm. In some embodiments, the second-wavelengthradiation includes ultraviolet light having a wavelength of about 400 nmor shorter. In other embodiments, the second-wavelength radiation is ofa longer wavelength than the first wavelength. In some such embodiments,the radiation of the longer second wavelength is at least 200 kW. Insome such embodiments, the radiation of the longer second wavelength isat least 300 kW. In some such embodiments, the radiation of the longersecond wavelength is at least 400 kW. In some such embodiments, theradiation of the longer second wavelength is at least 500 kW.

In some embodiments, the optical device includes a photonic-crystalfiber and the waveguide is the core of the photonic-crystal fiber thatsupports a fundamental mode field having a diameter of at least about 50microns and has a pump cladding having a diameter larger than that ofthe core. In some such embodiments, the fundamental mode field has adiameter of at least 60 microns. In some such embodiments, thefundamental mode field as a diameter of at least 70 microns. In somesuch embodiments, the fundamental mode field as a diameter of at least80 microns. In some such embodiments, the fundamental mode field as adiameter of at least 90 microns. In some such embodiments, thefundamental mode field as a diameter of at least 100 microns.

In some embodiments, the present invention provides an apparatus thatincludes an optical amplifier having a segmented fiber that includes afirst amplifying segment having a rare-earth-doped photonic-crystal coreand a second amplifying segment having a rare-earth-dopedphotonic-crystal core, wherein the first segment amplifies to a higherpower than the second segment and is non-contiguous with the secondsegment, a master-oscillator subsystem operable to obtain a seed lasersignal light and to optically couple the seed laser signal light intothe second segment, and a first optical-connector subassemblyoperatively coupled between the second segment and the first segment.

In some embodiments, the master-oscillator subsystem includes a firstoptical isolator and a first narrow-bandwidth band-pass optical filter,and is operable to obtain a narrow-linewidth, single-frequency seedlaser signal having a spectrally narrow signal bandwidth of less than 20GHz, optically coupled to the core of the second segment through thefirst optical isolator and the first narrow-bandwidth band-pass opticalfilter.

In some embodiments, the master-oscillator subsystem includes a firstoptical isolator and a first long-pass optical filter, and is operableto obtain a narrow-linewidth, single-frequency seed laser signal havinga spectrally narrow signal bandwidth of less than 20 GHz, opticallycoupled to the core of the second segment through the first opticalisolator and the first long-pass optical filter.

In some embodiments, the first optical-connector subassembly includes asecond narrow-bandwidth band-pass optical filter that is operable topass wavelengths of the narrow-linewidth, single-frequency seed lasersignal having a spectrally narrow signal bandwidth of less than 20 GHz,and to at least partially block wavelengths of amplified spontaneousemission (ASE) propagating back from the first segment.

In some embodiments, the first optical-connector subassembly includes asubstrate, an optical band-pass filter, and a dichroicmirror/beamsplitter that directs pump light into the second segment in acounter-propagating direction relative to signal light, and directssignal light from the second segment to the first segment through theoptical band-pass filter, wherein the filter and the mirror/beamsplitterare affixed to the substrate to form a unitized assembly.

In some embodiments, the first optical-connector subassembly includes asubstrate, an optical band-pass filter attached to the substrate, adichroic mirror/beamsplitter, attached to the substrate, that directspump light into the second segment in a counter-propagating directionrelative to signal light, and directs signal light from the secondsegment to the first segment through the optical band-pass filter, andat least one lens attached to the substrate, wherein at least one of thefilter, the mirror/beamsplitter and the lens is laser welded to thesubstrate to form a unitized assembly.

In some embodiments, the first optical-connector subassembly includes anenclosure having a substrate, an optical band-pass filter attached tothe substrate, a dichroic mirror/beamsplitter, attached to thesubstrate, that directs pump light into the second segment in acounter-propagating direction relative to signal light, and directssignal light from the second segment to the first segment through theoptical band-pass filter, and at least one lens attached to thesubstrate, wherein at least one of the filter, the mirror/beamsplitterand the lens is soldered to the enclosure to form a unitized assembly.

In some embodiments, the first optical-connector subassembly is anon-photonic-crystal bridge fiber having a multimode core having anumerical aperture (NA) higher than either the first or second segment'sphotonic-crystal core NA, and wherein a fundamental mode of thephotonic-crystal cores exhibits high spatial overlap with thosetransverse modes in the bridge fiber that exhibit a single centralmaximum.

In some embodiments, at least one of the first amplifying segment andthe second amplifying segment is capable of operation with asignal-output peak power of at least 500 kW.

In some embodiments, at least one of the first amplifying segment andthe second amplifying segment is capable of operation with asignal-output peak power of at least 2 megawatts (MW).

In some embodiments, at least one of the first amplifying segment andthe second amplifying segment is capable of operation with asignal-output-beam peak power of at least 500 kW and a beam-quality M²value of less than 1.5.

In some embodiments, at least one of the first amplifying segment andthe second amplifying segment is capable of generating linearlypolarized pulses having degree of polarization of at least 15 dB(wherein the degree of polarization is a value of ten times the log(base 10) of the ratio of optical power along the polarization axis tothe optical power along the orthogonal axis), a beam-quality M² value ofless than 1.5, and a peak power of at least about 100 kW.

In some embodiments, the present invention provides an apparatus havingan optical amplifier that includes first photonic-crystal means foramplifying optical signal pulses to obtain high-power optical signalpulses, second photonic-crystal means for amplifying optical signalpulses to obtain intermediate-power optical signal pulses, wherein thesecond means is non-contiguous with the first means, and means foroptically coupling the intermediate-power optical signal pulses to thefirst amplifying means from the second amplifying means, and means forgenerating optical seed signal pulses and for coupling the optical seedsignal pulses into the second amplifying means.

In some embodiments, the means for generating further includes means foroptically isolating the optical seed signal pulses, and means forfiltering the optical seed signal pulses before they enter the core ofthe second photonic-crystal means to obtain narrow-linewidth,single-frequency optical seed signal pulses having a spectrally narrowsignal bandwidth of less than 20 GHz.

In some embodiments, the means for optically coupling further includesmeans for filtering the intermediate-power optical signal pulses andsubstantially passing from the second photonic-crystal means toward thefirst photonic-crystal means those wavelengths corresponding to thenarrow-linewidth, single-frequency seed laser signal and substantiallyblocking other wavelengths of amplified spontaneous emission (ASE)propagating back from the first photonic-crystal means, wherein themeans for filtering the intermediate-power optical signal pulses arepart of a unitized assembly.

In some embodiments, the means for optically coupling further includesmeans for filtering the intermediate-power signal light andsubstantially passing from the second photonic-crystal means thosewavelengths corresponding to the narrow-linewidth, single-frequency seedlaser signal and substantially blocking other wavelengths of amplifiedspontaneous emission (ASE) propagating back from the firstphotonic-crystal means, means for directing pump light into the secondphotonic-crystal means in a counter-propagating direction relative tosignal light, and means for directing the filtered intermediate-powersignal light toward the first photonic-crystal means, wherein the meansfor filtering the intermediate-power optical signal pulses, the meansfor directing pump light, and the means for directing the filteredintermediate-power signal light are part of a unitized assembly.

In some embodiments, the present invention provides a method thatincludes providing an optical amplifier having a segmentedphotonic-crystal fiber that includes a higher-power first amplifyingsegment having a rare-earth-doped photonic-crystal core and alower-power second amplifying segment having a rare-earth-dopedphotonic-crystal core, obtaining a seed laser signal, optically couplingthe seed laser signal into the core of the second amplifying segment,amplifying the seed laser signal in the second amplifying segment toobtain intermediate-power signal light, operatively coupling theintermediate-power signal light to the first amplifying segment from thesecond amplifying segment, and amplifying the intermediate-power signallight in the first amplifying segment to obtain higher-power signallight.

Some embodiments of the method further include optically isolating theseed laser signal light, and narrow-bandwidth band-pass filtering theseed laser signal light before it enters the core of the second segmentto obtain a narrow-linewidth, single-frequency seed laser signal havinga spectrally narrow signal bandwidth of less than 20 GHz, and opticallycoupling the seed laser signal to the core of the second segment.

Some embodiments of the method further include optically isolating theseed laser signal, and long-pass optically filtering the seed lasersignal to obtain a narrow-linewidth, single-frequency seed laser signaloptically coupled to the core of the second segment.

Some embodiments of the method further include narrow-bandwidthband-pass filtering the intermediate-power signal light andsubstantially passing from the second segment toward the first segmentthose wavelengths corresponding to the narrow-linewidth,single-frequency seed laser signal and substantially blocking otherwavelengths of amplified spontaneous emission (ASE) propagating backfrom the first segment.

Some embodiments of the method further include narrow-bandwidthband-pass filtering the intermediate-power signal light andsubstantially passing from the second segment toward the first segmentthose wavelengths corresponding to the narrow-linewidth,single-frequency seed laser signal and substantially blocking otherwavelengths of amplified spontaneous emission (ASE) propagating backfrom the first segment, directing pump light into the second segment ina counter-propagating direction relative to signal light, and directingthe narrow-bandwidth band-pass filtered intermediate-power signal lighttoward the first segment within a unitized assembly.

Some embodiments of the method further include narrow-bandwidthband-pass filtering the intermediate-power signal light andsubstantially passing from the second segment toward the first segmentthose wavelengths corresponding to the narrow-linewidth,single-frequency seed laser signal and substantially blocking otherwavelengths of amplified spontaneous emission (ASE) propagating backfrom the first segment, focusing pump light into the second segment in acounter-propagating direction relative to the signal light, and focusingthe narrow-bandwidth band-pass filtered intermediate-power signal lightinto the core of the first segment within a unitized assembly.

In some embodiments, the operatively coupling the intermediate-powersignal light to the first amplifying segment from the second amplifyingsegment is performed through a non-photonic-crystal bridge fiber havinga multimode core having a numerical aperture (NA) higher than either thefirst or second segment's photonic-crystal core NA, and wherein afundamental mode of the photonic-crystal cores exhibits high spatialoverlap with those transverse modes in the bridge fiber that exhibit asingle central maximum.

In some embodiments, the amplifying of the intermediate-power signallight in the first amplifying segment generates a peak power of at least500 kW. In some embodiments, the amplifying of the intermediate-powersignal light in the first amplifying segment generates a peak power ofat least 2 MW.

In some embodiments, the amplifying of the intermediate-power signallight in the first amplifying segment generates a peak power of at least500 kW and a beam-quality M² value of less than 1.5.

In some embodiments, the amplifying of the intermediate-power signallight in the first amplifying segment generates linearly polarizedpulses having degree of polarization of at least 15 dB (wherein thedegree of polarization is a value of ten times the log (base 10) of theratio of optical power along the polarization axis to the optical poweralong the orthogonal axis), a beam-quality M² value of less than 1.5,and a peak power of at least about 100 kW.

Optical Hollow-Core Delivery Fiber and Termination and Associated Method

Some embodiments of the invention provide an apparatus that includes ahollow-core photonic-crystal fiber (HCPCF) configured to receive anoptical beam such that the optical beam is coupled into its hollow coreand is guided therein with low optical loss along the whole length ofthis fiber and wherein a first end of the fiber is cleaved to form anend facet and is sealed to a first closed-end connector attached to thefiber without the aid of epoxy adhesives or other organic compounds.

In some embodiments, the first closed-end connector includes a slantedend window and a hollow cavity between the end facet of the HCPCF andthe end window.

In some embodiments, the first closed-end connector includes anend-mounted collimating lens window and a hollow cavity between the endfacet of the HCPCF and the end window.

Some embodiments further include a second closed-end connector attachedto a second end of the HCPCF, wherein the second closed-end connectorincludes an end-mounted focusing lens window and a hollow cavity betweenthe end facet of the HCPCF and the end window.

In some embodiments, the first closed-end connector includes a hollowcavity that is laser welded around the first end of the HCPCF.

In some embodiments, the first closed-end connector includes a hollowcavity that attached around the first end of the HCPCF by heatshrinking.

In some embodiments, the first closed-end connector includes a hollowcavity that soldered around the first end of the HCPCF.

In some embodiments, the first closed-end connector includes a hollowglass ferrule that is laser welded around the first end of the HCPCF,and an end window laser welded to the ferrule.

Some embodiments further include a photonic-crystal amplifier device(PCAD) having a core diameter of at least 100 microns and configured toamplify dispersively stretched pulses, wherein the PCAD is opticallycoupled to deliver high-power optical pulses to the HCPCF, and whereinthe HCPCF has dispersive properties tailored to recompress the amplifiedstretched pulses.

In some embodiments, the first connector is an enclosure that has ahollow first end with an internal surface that is laser-welded alongpart of its length to an outer surface of the HCPCF in proximity of thefirst end of the fiber. In some such embodiments, the connector has asealed second end such that the first end of the fiber is fully enclosedand sealed within an inner space of the attached connector. In someembodiments, the sealed second end of the first connector is terminatedby an optical window located at a distance from the end facet of theHCPCF and oriented at a non-perpendicular angle with respect a centralaxis of an optical beam emitted from the end facet, such that the innerspace of the connector forms a hollow, sealed region. This seal protectsthe exposed holes in the end facet of the HCPCF from contamination fromexternal impurities. The non-perpendicular angle between the connector'swindow and the optical axis of the beam exiting the HCPCF facet reducesthe optical feedback into the fiber by minimizing the fraction of lightexiting the fiber that is back-reflected by the window and coupled backinto the HCPCF. In some embodiments, at least one surface of the opticalwindows terminating the first connector is anti-reflection coated forwavelengths of the optical beam propagating within the HCPCF.

In some embodiments, the sealed second end of the first connector isterminated by a lens-shaped optical window located at a distance fromthe end facet of the HCPCF, such that the inner space of the connectorforms a hollow, sealed region. In some embodiments, the lens-shapedoptical window is shaped and located to provide collimation for theoptical beam exiting the HCPCF and propagating through the lens-shapedwindow. In other embodiments, the lens-shaped optical window is shapedand located to provide focusing for the optical beam propagating throughthe window and then entering the HCPCF.

In some embodiments, the apparatus further includes at least a first anda second rare-earth-doped photonic crystal fiber (REDPCF), wherein theHCPCF is operably coupled between the first REDPCF and second REDPCFsuch that an optical output beam from the first rare-earth-dopedphotonic-crystal fiber is coupled into the hollow-core photonic-crystalfiber and propagates therein along the whole length of the hollow coreand exits the hollow core and is coupled into the secondrare-earth-doped fiber, and wherein each piece of hollow-corephotonic-crystal fiber constitutes an optical-beam-delivery mediumbetween pieces of rare-earth-doped fiber positioned at a distance withrespect to each other and the optical beam propagating in each piece ofhollow-core photonic-crystal fiber undergoes negligible opticalnonlinear effect as it propagates through the whole hollow-core fiberlength.

In some embodiments, the HCPCF is positioned between and opticallycouples a piece of rare-earth-doped solid-body fiber and a piece ofrare-earth-doped photonic-crystal fiber. In some embodiments, the HCPCFis positioned between and optically couples a first piece ofrare-earth-doped solid-body fiber and a second piece of rare-earth-dopedsolid-body fiber. In some embodiments, the HCPCF is positioned betweenand optically couples a rare-earth-doped photonic-crystal rod and apiece of rare-earth-doped photonic-crystal fiber. In some embodiments,the HCPCF is positioned between and optically couples a rare-earth-dopedphotonic-crystal rod (REDPCR) and a piece of rare-earth-doped solid-bodyfiber (REDPCF). In some embodiments, the HCPCF is positioned between andoptically couples two rare-earth-doped photonic-crystal rods.

In some embodiments, the present invention provides an apparatus havinga hollow-core photonic-crystal fiber (HCPCF) having a first end of theHCPCF cleaved to form an end facet, and a first closed-end connectorhaving an optically-transmissive port, and means for connecting thefirst end into the first closed-end connector attached to the first endof the HCPCF without the aid of epoxy adhesives or other organiccompounds.

In some embodiments, the first closed-end connector includes a slantedend window and a hollow cavity between the end facet of the HCPCF andthe end window.

In some embodiments, the first closed-end connector includes anend-mounted collimating lens window and a hollow cavity between the endfacet of the HCPCF and the end window.

Some embodiments further include a second closed-end connector, whereinthe second closed-end connector includes an end-mounted focusing lenswindow and a hollow cavity between the end facet of the HCPCF and theend window, and means for attaching the second closed-end connector to asecond end of the HCPCF.

In some embodiments, the first closed-end connector includes a hollowcavity, wherein the connecting further includes laser welded means forconnecting the first closed-end connector around the first end of theHCPCF.

In some embodiments, the present invention provides method that includesproviding a hollow-core photonic-crystal fiber (HCPCF) having a firstend of the HCPCF cleaved to form an end facet, and a first closed-endconnector having an optically-transmissive port, and connecting thefirst end into the first closed-end connector attached to the first endof the HCPCF without the aid of epoxy adhesives or other organiccompounds.

In some embodiments, the first closed-end connector includes a slantedend window and a hollow cavity between the end facet of the HCPCF andthe end window.

In some embodiments, the first closed-end connector includes anend-mounted collimating lens window and a hollow cavity between the endfacet of the HCPCF and the end window.

Some embodiments further include providing a second closed-endconnector, wherein the second closed-end connector includes anend-mounted focusing lens window and a hollow cavity between the endfacet of the HCPCF and the end window, and attaching the secondclosed-end connector to a second end of the HCPCF.

In some embodiments, the first closed-end connector includes a hollowcavity, wherein the connecting further includes laser welding the firstclosed-end connector around the first end of the HCPCF.

In some embodiments, the first closed-end connector includes a hollowcavity, wherein the connecting further includes attaching the firstclosed-end connector around the first end of the HCPCF by heatshrinking.

In some embodiments, the first closed-end connector includes a hollowcavity, wherein the connecting further includes soldering the firstclosed-end connector around the first end of the HCPCF.

In some embodiments, the first closed-end connector includes a hollowglass ferrule and the method further includes laser welding the firstclosed-end connector around the first end of the HCPCF, and laserwelding an end window to the ferrule.

Some embodiments further include providing a photonic-crystal amplifierdevice (PCAD) having a core diameter of at least 100 microns,configuring the PCAD to amplify dispersively stretched pulses, opticallycoupling the PCAD to deliver high-power optical pulses to the HCPCF, andconfiguring the HCPCF to have dispersive properties tailored torecompress the amplified stretched pulses.

Method and Apparatus for Long-Range LIDAR and Active Imaging withOptical Output from a Photonic-Crystal Rod

In some embodiments, the present invention provides an apparatus thatincludes a first photonic-crystal optical device that includes a firstwaveguide that has a diameter of at least about 40 microns, maintains asingle transverse mode, and is operable to directly generate opticalpulses having a peak power of at least 500 kW, a spectral linewidth of 1nm or less, a pulse-to-CW-background ratio of at least 20 dB, and a beamquality M²<2 at a wavelength of 1.5 microns or longer (thereby meetingthe requirement for “eye safe” operation), and is operated as acomponent of an optical transmitter within a device that performslong-range active optical imaging, the list of applicable devicesincluding Light Detector and Ranging (LIDAR) devices that include atleast one laser.

In some embodiments, the photonic crystal device is a photonic-crystalfiber having a core doped with Erbium and having a core diameter largerthan 40 microns. In some embodiments, the photonic-crystal device is aphotonic-crystal fiber having a core codoped with Erbium and Ytterbiumand having a core diameter larger than 40 microns. In some embodiments,the photonic crystal device is a photonic crystal fiber having a coredoped with Thulium and having a core diameter larger than 40 microns. Insome embodiments, the photonic-crystal device is a photonic-crystalfiber having a core codoped with Thulium and Holmium and having a corediameter larger than 40 microns.

In some embodiments, the photonic-crystal device is a photonic-crystalrod having a core doped with Erbium and having a core diameter largerthan 60 microns. In some embodiments, the photonic-crystal device is aphotonic-crystal rod having a core codoped with Erbium and Ytterbium andhaving a core diameter larger than 60 microns. In some embodiments, thephotonic-crystal device is a photonic-crystal rod having a core dopedwith Thulium and having a core diameter larger than 60 microns. In someembodiments, the photonic-crystal device is a photonic-crystal rodhaving a core codoped with Thulium and Holmium and having a corediameter larger than 60 microns.

In some embodiments, the present invention provides an apparatusincluding an optical amplifier having a segmented photonic-crystal fiber(PCF) that includes a first amplifying segment having a rare-earth-dopedphotonic-crystal core and a second amplifying segment having arare-earth-doped photonic-crystal core, a master oscillator operable togenerate a seed-laser signal, and a first optical connector subassemblyoperatively coupled between the first segment and the second segment,wherein the segmented PCF receives in input and emits as output anoptical beam of wavelength of 1.5 microns or longer, the output of theapparatus as a whole including optical pulses having a peak power of atleast 500 kW, a spectral linewidth of 1 nm or less, apulse-to-CW-background ratio of at least 20 dB, and a beam quality M²<2at a wavelength of 1.5 microns or longer (thereby meeting therequirement for “eye safe” operation), and wherein the apparatus isoperated as a component of an optical transmitter device that performslong-range active optical imaging.

In some embodiments, the apparatus further includes a plurality ofphotonic-crystal optical devices each producing an output beam, and aplurality of dispersive optical elements (e.g., diffraction gratings)arranged to combine the plurality of output beams into a single opticalbeam, the apparatus as a whole being operated as a component of anoptical transmitter, wherein each photonic-crystal device is arare-earth-doped photonic-crystal fiber having core diameter of at least40 microns and is operated as an optical pulse amplifier so as todirectly generate optical pulses of peak power of 500 kW or more andspectral linewidth of 1 nm or less and pulse-to-CW-background ratio of20 dB or more and beam quality M²<2 at a wavelength of 1.5 microns orlonger, thereby meeting the requirement for “eye safe” operation, and isoperated as a component of an optical transmitter within a device thatperforms long-range active optical imaging, the list of applicabledevices including Light Detector and Ranging (LIDAR) devices thatinclude at least one laser.

In some embodiments, the present invention provides an apparatusincluding an optical transmitter component configured for use in along-range optical measuring device, the component including a signallaser that emits a laser signal, and a photonic-crystal opticalamplifier device operatively coupled to receive the signal laser signal,the photonic-crystal optical amplifier device having a first signalwaveguide that has a diameter of at least about 40 microns, maintains asingle transverse mode, and is operable to directly generate an outputsignal of optical pulses having a peak power of at least 500 kW, aspectral linewidth of 1 nm or less, a pulse-to-CW-background ratio of atleast 20 dB, and a beam-quality M² value of less than 2 at a wavelengthof 1.5 microns or longer.

Some embodiments further include a scanning component that scans atleast a portion of the output signal across an area, an imager componentthat obtains an image signal representing at least a portion of thescanned area, and a display component that displays an image based onthe image signal.

Some embodiments further include a scanning component that scans atleast a portion of the output signal across an area, an imager componentthat obtains an image signal representing at least a portion of thescanned area, a distance-determining component that determines adistance based on a time-of-flight measurement of at least one of theoptical pulses, and a display component that displays an image based onthe image signal, wherein the image also provides an indication of thedistance determined by the distance-determining component.

Some embodiments further include a distance-determining component thatdetermines a distance based on a measurement of at least one of theoptical pulses, and a display component that displays an indication ofthe distance determined by the distance-determining component.

Some embodiments further include a beam-expanding component that expandsat least a portion of the output signal to illuminate an area, ashuttered imager component that obtains an image signal representing atleast a portion of the illuminated area during a pulse, and a displaycomponent that displays an image based on the image signal.

Some embodiments further include a scanning component that scans atleast a portion of the output signal across an area, wherein temporallydifferent pulses illuminate various portions of the area, an imagercomponent having a two-dimensional array of pixels that obtain an imagesignal representing at least a portion of the scanned area, and whereindifferent subsets of the array of pixels, each having fewer than all thepixels, obtain image information from different pulses, and a displaycomponent that displays an image based on the image information.

In some embodiments, the photonic-crystal optical amplifier device is anoptical fiber, and the first signal waveguide is a core of the fiber,the core exhibiting a numerical aperture defined by photonic-crystalstructures.

In some embodiments, the present invention provides method that includesobtaining a laser signal having pulses, coupling the pulsed laser signalto a first photonic-crystal optical amplifier device having a firstsignal waveguide that has a diameter of at least about 40 microns, andwhich maintains a single transverse mode, and amplifying the pulsedlaser signal to generate an output signal of optical pulses having apeak power of at least 500 kW, a spectral linewidth of 1 nm or less, apulse-to-CW-background ratio of at least 20 dB, and a beam-quality M²value of less than 2 at a signal wavelength of 1.5 microns or longer.

Some embodiments further include scanning at least a portion of theoutput signal across an area, generating an image signal representing atleast a portion of the scanned area, and displaying an image based onthe image signal.

Some embodiments further include scanning at least a portion of theoutput signal across an area, generating an obtains an image signalrepresenting at least a portion of the scanned area, determining adistance based on a time-of-flight measurement of at least one of theoptical pulses, and displaying an image based on the image signal,wherein the image also provides an indication of the distance determinedby the distance-determining component.

Some embodiments further include determining a distance based on ameasurement of at least one of the optical pulses, and displaying anindication of the distance determined by the distance-determiningcomponent.

Some embodiments further include expanding at least a portion of theoutput signal to illuminate an area, shuttered-imaging the area toobtain an image signal representing at least a portion of theilluminated area during a pulse, and displaying an image based on theimage signal.

Some embodiments further include scanning at least a portion of theoutput signal across an area, wherein temporally different pulsesilluminate various portions of the area, generating an image signalrepresenting a two-dimensional array of pixels of at least a portion ofthe scanned area, and wherein different subsets of the array of pixels,each having fewer than all the pixels, represent image information fromdifferent pulses, and displaying an image based on the imageinformation.

In some embodiments, the present invention provides an apparatus thatincludes an optical transmitter component configured for use in along-range optical measuring device, the component including

a signal laser that emits a pulsed laser signal, and photonic-crystalmeans for amplifying the pulsed laser signal while maintaining a singletransverse mode to generate an output signal of optical pulses having apeak power of at least 500 kW, a spectral linewidth of 1 nm or less, apulse-to-CW-background ratio of at least 20 dB, and a beam-quality M²value of less than 2 at a signal wavelength of 1.5 microns or longer.

Some embodiments further include means for scanning at least a portionof the output signal across an area, means for generating an imagesignal representing at least a portion of the scanned area, and meansfor displaying an image based on the image signal.

Some embodiments further include means for scanning at least a portionof the output signal across an area, means for generating an obtains animage signal representing at least a portion of the scanned area, meansfor determining a distance based on a time-of-flight measurement of atleast one of the optical pulses, and means for displaying an image basedon the image signal, wherein the image also provides an indication ofthe distance determined by the distance-determining component.

Some embodiments further include means for determining a distance basedon a measurement of at least one of the optical pulses, and means fordisplaying an indication of the distance determined by thedistance-determining component.

Some embodiments further include means for expanding at least a portionof the output signal to illuminate an area, means for shuttered-imagingthe area to obtain an image signal representing at least a portion ofthe illuminated area during a pulse, and means for displaying an imagebased on the image signal.

Some embodiments further include means for scanning at least a portionof the output signal across an area, wherein temporally different pulsesilluminate various portions of the area, means for generating an imagesignal representing a two-dimensional array of pixels of at least aportion of the scanned area, and wherein different subsets of the arrayof pixels, each having fewer than all the pixels, represent imageinformation from different pulses, and means for displaying an imagebased on the image information.

In some embodiments, the photonic-crystal means for amplifying includesan optical fiber having a core exhibiting a numerical aperture definedby photonic-crystal structures.

In some embodiments, the photonic-crystal means for amplifying includesa photonic crystal fiber having a core diameter larger than 40 microns.

In some embodiments, the photonic-crystal means for amplifying includesa photonic crystal rod having a core diameter larger than 60 microns.

In some embodiments, the photonic-crystal means for amplifying includesa photonic crystal rod having a core is doped with Erbium.

In some embodiments, the photonic-crystal means for amplifying includesa photonic crystal rod having a core is codoped with Erbium andYtterbium.

In some embodiments, the photonic-crystal means for amplifying includesa photonic crystal rod having a core is doped with Thulium.

In some embodiments, the photonic-crystal means for amplifying includesa photonic crystal rod having a core is codoped with Thulium andHolmium.

Some embodiments further include at least one other signal laser thateach emits a pulsed laser signal, and at least one otherphotonic-crystal means for amplifying the at least one other pulsedlaser, and means for spectral-beam combining output signals from theplurality of means for amplifying into a single optical beam.

Method and Apparatus for Ultra-Violet-Wavelength Laser-InducedFluorescence (UV-LIF) Using Optical Output from a Photonic-Crystal Rod

In some embodiments, the present invention provides an apparatus thatincludes a first photonic-crystal optical device that includes a firstwaveguide that has a diameter of at least 40 microns and maintains asingle transverse mode, and at least one wavelength-conversion opticalmedium (e.g., non-linear frequency doublers, optical parametricoscillators and the like) operable to receive high-peak-power inputradiation having a first wavelength from the first waveguide of thefirst photonic-crystal device and to generate radiation having a peakpower of at least about 100 kW and of a different second wavelengththrough wavelength conversion, wherein the apparatus is operated as anoptical transmitter within a device that performsultra-violet-wavelength laser-induced-fluorescence (UV-LIF) detectionand wherein the optical beam of shorter wavelength generated by theapparatus through the wavelength conversion optical media falls in the200-400-nm wavelength range and its peak power is of at least 100 kW andthe optical beam can be directed towards an airborne cloud carryinginorganic chemicals, organic chemicals and/or biological compounds,these chemicals or compounds being able to absorb light at thewavelength of the optical beam and release light at a differentwavelength by fluorescence and this emitted light can be received anddetected by one or more optical devices that are operated along with thetransmitter within the UV-LIF device.

In some embodiments, the photonic-crystal optical device includes asegmented photonic-crystal fiber that includes a first amplifyingsegment having a rare-earth-doped photonic-crystal core and a secondamplifying segment having a rare-earth-doped photonic-crystal core, amaster oscillator operable to generate a seed-laser signal, and a firstoptical connector subassembly operatively coupled between the firstsegment and the second segment.

In some embodiments, the present invention provides an apparatus thatincludes a first photonic-crystal rod (PCR), having rare-earth-dopedcore with a diameter of at least 40 microns and an external diameter ofat least 1 mm such that the rod is therefore thick enough tosubstantially and readily hold its shape when released.

In some embodiments, the photonic-crystal optical device includes afirst segmented photonic-crystal rod (PCR), having rare-earth-doped corewith a diameter of at least 40 microns and an external diameter of atleast 1 mm such that the rod is therefore thick enough to substantiallyand readily hold its shape when released, and wherein thephotonic-crystal rod is configured as at least two segments that areserially encountered along a signal's optical path. In some embodiments,each two consecutive PCR segments in the series are joined using a pieceof bridge optical fiber having an outer diameter different from that ofthe PCR segments and its ends spliced to the ends of the PCR segments soas to form a chain of alternating PCR segments and bridge-fibersegments, wherein the chain is operable as a continuous opticalwaveguide. In some such embodiments, each bridge fiber is fused at itsends to its two PCR segments. In some such embodiments, each bridgefiber is laser-welded to its two PCR segments. In some such embodiments,the laser welding acts to seal ends of the PCR's core's holes.

In some embodiments, the apparatus is an enabling optical transmitterfor performing long-range detection of airborne chemicals and biologicalagents.

In some embodiments, the present invention provides an apparatus thatincludes an optical transmitter component configured for use in along-range optical measuring device, the component including a signallaser that emits a laser signal, and a first photonic-crystal opticaldevice operatively coupled to receive the signal laser signal, thephotonic-crystal optical amplifier device including a signal waveguidethat has a diameter of at least 40 microns and maintains a singletransverse mode, and at least one wavelength-conversion optical mediumoperable to receive high-peak-power input radiation having a firstwavelength from the signal waveguide of the first photonic-crystaldevice and to generate radiation having a peak power of at least about100 kW and a second wavelength that is shorter than 400 nm throughwavelength conversion.

In some embodiments, the wavelength-conversion optical medium includesat least one non-linear frequency doubler.

In some embodiments, the wavelength-conversion optical medium includesan optical parametric oscillator.

Some embodiments further include a detector configured to detectultra-violet-wavelength laser-induced-fluorescence (UV-LIF), wherein thefirst photonic-crystal optical device is operated as part of an opticaltransmitter within a device that directs the optical beam towards anairborne cloud material to be analyzed, and an analysis unit configuredto detect specific substances.

In some embodiments, the analysis unit is configured to analyzeinorganic chemicals.

In some embodiments, the analysis unit is configured to analyze organicchemicals.

In some embodiments, the analysis unit is configured to analyzebiological compounds.

In some embodiments, the present invention provides a method thatincludes obtaining a pulsed laser signal, amplifying the signal lasersignal using a photonic-crystal optical amplifier device that includes asignal waveguide that has a diameter of at least 40 microns andmaintains a single transverse mode and generates, high-peak-power pulsedradiation, and wavelength-converting the high-peak-power pulsedradiation having a first wavelength from the signal waveguide of thephotonic-crystal device to generate radiation having a peak power of atleast about 100 kW and a second wavelength that is shorter than 400 nm.

In some embodiments of the method, the wavelength-converting includes atleast one non-linear frequency doubling.

In some embodiments, the wavelength-converting includes opticalparametric oscillating.

Some embodiments further include optically transmitting the optical beamtowards an airborne cloud material to be analyzed, detectingultra-violet-wavelength laser-induced-fluorescence (UV-LIF), andanalyzing the detected UV_LIF to detect specific substances.

In some embodiments, the analyzing is configured to analyze inorganicchemicals.

In some embodiments, the analyzing is configured to analyze organicchemicals.

In some embodiments, the analyzing is configured to analyze biologicalcompounds.

In some embodiments, the present invention provides an apparatus thatincludes an optical transmitter component configured for use in along-range optical measuring device, the component including a signallaser that emits a laser signal, photonic-crystal means for amplifyingthe signal laser signal having a signal waveguide that has a diameter ofat least 40 microns and maintains a single transverse mode and thatgenerates high-peak-power pulsed radiation having a first wavelength,and means for wavelength-converting the high-peak-power pulsed radiationto generate radiation having a peak power of at least about 100 kW and asecond wavelength that is wavelength shorter than 400 nm.

In some embodiments, the means for wavelength-converting includes atleast one non-linear means for frequency doubling.

In some embodiments, the means for wavelength-converting includes anoptical parametric oscillator.

Some embodiments further include means for optically transmitting theoptical beam towards an airborne cloud material to be analyzed, meansfor detecting ultra-violet-wavelength laser-induced-fluorescence(UV-LIF), and means for analyzing the detected UV_LIF to detect specificsubstances.

In some embodiments, the means for analyzing is configured to analyzeinorganic chemicals.

In some embodiments, the means for analyzing is configured to analyzeorganic chemicals.

In some embodiments, the means for analyzing is configured to analyzebiological compounds.

Chirped Pulse Amplifiers Using Optical Output from a Photonic-CrystalRod (PCR) and Associated Method

In some embodiments, the present invention provides an apparatus thatincludes a first photonic-crystal rod (PCR), having rare-earth-dopedcore with a diameter of at least 50 microns and an external diameter ofat least 1 mm such that the rod is therefore thick enough tosubstantially and readily hold its shape when released, wherein therare-earth-doped photonic-crystal rod (REDPCR) receives, as signalinput, spectrally broad chirped optical pulses having a FWHM spectrallinewidth of at least 10 nm and a duration of at least 100 ps, andamplifies the pulses to obtain an output pulse energy of at least 0.5mJ, peak power of at least 1 MW, and beam quality M²<1.5. In someembodiments, the spectrally broad optical input pulses are obtained bytemporally stretching and spectrally chirping optical pulses havingduration of 100 ps or less emitted from an external optical source. Insome embodiments, the rare-earth-doped photonic-crystal rod includesinternal elements (e.g., stress rods) that induce birefringence in thecore thereby ensuring that the optical beam emitted by thephotonic-crystal rod is linearly polarized and the degree ofpolarization is at least 15 dB. In some embodiments, the optical beamemitted by at least one segment is linearly polarized with a degree ofpolarization being at least 16 dB. In some embodiments, the optical beamemitted by at least one segment is linearly polarized with a degree ofpolarization being at least 17 dB. In some embodiments, the optical beamemitted by at least one segment is linearly polarized with a degree ofpolarization being at least 18 dB. In some embodiments, the optical beamemitted by at least one segment is linearly polarized with a degree ofpolarization being at least 19 dB. In some embodiments, the optical beamemitted by at least one segment is linearly polarized with a degree ofpolarization being at least 20 dB.

In some embodiments, the present invention provides an apparatus havinga plurality of optical devices including photonic-crystal fibers,photonic-crystal rods, and solid-body fibers, each of which has arare-earth-doped core, and which are arranged in series and areseparated by subassemblies and operated as optical amplifiers having asegmented photonic-crystal fiber that includes a first amplifyingsegment having a rare-earth-doped photonic-crystal core and a secondamplifying segment having a rare-earth-doped photonic-crystal core, amaster oscillator operable to generate a seed-laser signal, and a firstoptical connector subassembly operatively coupled between the firstsegment and the second segment. In some embodiments, themaster-oscillator subsystem includes a first optical isolator and afirst narrow-bandwidth band-pass optical filter, and is operable togenerate a narrow-linewidth, single-frequency seed-laser signal coupledto the core of the first segment through the first optical isolator andthe first narrow-bandwidth band-pass optical filter. In someembodiments, the master-oscillator subsystem includes a first opticalisolator and a first long-pass optical filter, and is operable togenerate a narrow-linewidth, single-frequency seed-laser signal coupledto the core of the first segment through the first optical isolator andthe first long-pass optical filter.

In some embodiments, the apparatus receives, as signal input, spectrallybroad chirped optical pulses having an FWHM spectral linewidth of atleast 10 nm and a duration of at least 100 ps, the output from the finalamplifier having spectrally broad chirped optical pulses having an FWHMspectral linewidth of at least 10 nm and a duration of at least 100 ps,and an output pulse energy of at least 0.5 mJ, peak power of at least 1MW, and beam quality M²<1.5.

In some embodiments, the output optical beam from the final amplifier islinearly polarized and the degree of polarization is at least 15 dB.

In some embodiments, the present invention provides a method thatincludes a first photonic-crystal rod (PCR), having rare-earth-dopedcore with a diameter of at least 50 microns and an external diameter ofat least 1 mm such that the rod is therefore thick enough tosubstantially hold its shape when released, wherein the rare-earth-dopedphotonic-crystal rod (REDPCR) receives, as signal input, spectrallybroad chirped optical pulses having a FWHM spectral linewidth of atleast 10 nm and a duration of 1 ns of less, and amplifies the pulses toobtain an output pulse energy of at least 0.5 mJ, peak power of at least1 megawatts (MW), and beam-quality M² value of less than 1.5.

In some embodiments, the spectrally broad optical input pulses areobtained by temporally stretching and spectrally chirping optical pulseshaving duration of 100 ps or less emitted from an external opticalsource.

In some embodiments, the rare-earth-doped photonic-crystal rod includesinternal stress rods that induce birefringence in the core therebyensuring that the optical beam emitted by the photonic-crystal rod islinearly polarized and the degree of polarization is at least 15 dB.

In some embodiments, the rare-earth-doped photonic-crystal rod includesinternal elements that induce birefringence in the core thereby ensuringthat the optical beam emitted by the photonic-crystal rod is linearlypolarized and the degree of polarization is at least 15 dB.

In some embodiments, the peak power is at least 4 MW.

Some embodiments further include at least one solid-body fiber, at leastone seed laser subsystem configured to emit chirped pulses, and at leastone photonic-crystal fiber, wherein the first photonic-crystal rod, thesolid-body fiber, the laser subsystem, and the photonic-crystal fiberare parts of a series of optical components separated by free-spaceoptical subassemblies that provide pump light into the series of opticalcomponents.

Some embodiments further include at least one solid-body fiber, at leastone seed laser subsystem configured to emit chirped pulses, and at leastone photonic-crystal fiber, wherein the first photonic-crystal rod, thesolid-body fiber, the laser subsystem, and the photonic-crystal fiberare parts of a series of optical components separated by free-spaceoptical subassemblies that provide pump light into the series of opticalcomponents, and wherein the rare-earth-doped photonic-crystal rodincludes internal elements that induce birefringence in the core therebyensuring that the optical beam emitted by the photonic-crystal rod islinearly polarized and the degree of polarization is at least 15 dB.

In some embodiments, the present invention provides a method thatincludes providing a first photonic-crystal rod (PCR) havingrare-earth-doped core with a diameter of at least 50 microns and anexternal diameter of at least 1 mm such that the rod is therefore thickenough to substantially hold its shape when released, optically couplingas signal input into the rare-earth-doped photonic-crystal rod (REDPCR)spectrally broad chirped optical pulses having a FWHM spectral linewidthof at least 10 nm and a duration of 1 ns of less, and amplifying thepulses in the REDPCR to obtain an output pulse energy of at least 0.5mJ, peak power of at least 1 MW, and beam-quality M² value of less than1.5.

Some embodiments further include obtaining optical pulses havingduration of 100 ps or less, and temporally stretching and spectrallychirping the optical pulses to produce the spectrally broad opticalinput pulses.

Some embodiments further include ensuring that the optical beam emittedby the photonic-crystal rod is linearly polarized and the degree ofpolarization is at least 15 dB by including, in the rare-earth-dopedphotonic-crystal rod, internal stress rods that induce birefringence inthe core of the rare-earth-doped photonic-crystal rod.

Some embodiments further include ensuring that the optical beam emittedby the photonic-crystal rod is linearly polarized and the degree ofpolarization is at least 15 dB by including, in the rare-earth-dopedphotonic-crystal rod, internal elements that induce birefringence in thecore of the rare-earth-doped photonic-crystal rod.

In some embodiments, the amplifying generates peak power of at least 4MW.

Some embodiments further include serially connecting at least onesolid-body fiber, at least one seed laser subsystem configured to emitchirped pulses, and at least one photonic-crystal fiber, as parts of aseries of optical components separated by free-space opticalsubassemblies, and injecting pump light into the series of opticalcomponents through the free-space optical subassemblies.

Some embodiments further include serially connecting at least onesolid-body fiber, at least one seed laser subsystem configured to emitchirped pulses, and at least one photonic-crystal fiber, as parts of aseries of optical components separated by free-space opticalsubassemblies, injecting pump light into the series of opticalcomponents through the free-space optical subassemblies, and ensuringthat the optical beam emitted by the photonic-crystal rod is linearlypolarized and the degree of polarization is at least 15 dB by including,in the rare-earth-doped photonic-crystal rod, internal elements thatinduce birefringence in the core of the rare-earth-dopedphotonic-crystal rod.

In some embodiments, the present invention provides an apparatus thatincludes photonic-crystal-rod means for amplifying, the means foramplifying having rare-earth-doped means for waveguiding with a diameterof at least 50 microns and means for substantially holding the shape ofthe means for amplifying when released, means for optically coupling assignal input into the rare-earth-doped photonic-crystal rod (REDPCR)spectrally broad chirped optical pulses having a FWHM spectral linewidthof at least 10 nm and a duration of 1 ns of less, and means for pumpingthe means for amplifying in order to obtain an output pulse energy of atleast 0.5 mJ, peak power of at least 1 MW, and beam-quality M² value ofless than 1.5.

Some embodiments further include means for obtaining optical pulseshaving duration of 100 ps or less, and means for temporally stretchingand spectrally chirping the optical pulses to produce the spectrallybroad optical input pulses.

Some embodiments further include means for ensuring that the opticalbeam emitted by the photonic-crystal rod is linearly polarized and thedegree of polarization is at least 15 dB including internal means forstressing the means for waveguiding.

Some embodiments further include means for ensuring that the opticalbeam emitted by the photonic-crystal rod is linearly polarized and thedegree of polarization is at least 15 dB including means for inducingbirefringence in the means for waveguiding.

In some embodiments, the means for amplifying generates peak power of atleast 4 MW.

Some embodiments further include means for serially connecting at leastone solid-body fiber, at least one seed laser subsystem configured toemit chirped pulses, and at least one photonic-crystal fiber, as partsof a series of optical components separated by free-space opticalsubassemblies, and means for injecting pump light into the series ofoptical components through the free-space optical subassemblies.

Some embodiments further include means for serially connecting at leastone solid-body fiber, at least one seed laser subsystem configured toemit chirped pulses, and at least one photonic-crystal fiber, as partsof a series of optical components separated by free-space opticalsubassemblies, means for injecting pump light into the series of opticalcomponents through the free-space optical subassemblies, and means forensuring that the optical beam emitted by the photonic-crystal rod islinearly polarized and the degree of polarization is at least 15 dBincluding means for inducing birefringence in the means for waveguiding.

Method and Apparatus for Spectral-Beam Combining of Megawatt-Peak-PowerBeams from Photonic-Crystal Rods

In some embodiments, the present invention provides an apparatus thatincludes a plurality of photonic-crystal optical devices, each includinga first waveguide that has a diameter of about 40 microns or more andmaintains a single transverse mode, wherein the plurality ofphotonic-crystal optical devices are arranged side-by-side and operatedsuch that each device can emit a pulsed optical beam of a wavelengthdifferent from that of the other devices therein, the apparatus furtherincluding at least one external dispersive optical element that receivesthe emitted optical beams and combines them into a single combined beamhaving M²<2 and peak power of at least 2 MW, the peak power of thecombined beam being at least 50% of the sum of the peak powers in theoptical beams.

In some embodiments, the combined beam exhibits peak power of at least 3MW. In some embodiments, the combined beam exhibits peak power of atleast 4 MW. In some embodiments, the combined beam exhibits peak powerof at least 5 MW. In some embodiments, the combined beam exhibits peakpower of at least 6 MW. In some embodiments, the combined beam exhibitspeak power of at least 7 MW. In some embodiments, the combined beamexhibits peak power of at least 8 MW. In some embodiments, the combinedbeam exhibits peak power of at least 9 MW. In some embodiments, thecombined beam exhibits peak power of at least 10 MW. In someembodiments, the combined beam exhibits peak power of at least 20 MW. Insome embodiments, the combined beam exhibits peak power of at least 30MW. In some embodiments, the combined beam exhibits peak power of atleast 40 MW. In some embodiments, the combined beam exhibits peak powerof at least 50 MW. In some embodiments, the combined beam exhibits peakpower of at least 60 MW. In some embodiments, the combined beam exhibitspeak power of at least 70 MW. In some embodiments, the combined beamexhibits peak power of at least 80 MW. In some embodiments, the combinedbeam exhibits peak power of at least 90 MW. In some embodiments, thecombined beam exhibits peak power of at least 100 MW.

In some embodiments, each photonic-crystal optical device furtherincludes a respective master oscillator operable to emit an optical beamof a different wavelength than those of the other master oscillators, anoptical isolator, and a narrow-bandwidth filter, wherein the masteroscillator is operable to generate a narrow-linewidth seed-laser signaloperably coupled to the core of the respective photonic-crystal opticaldevice through the first optical isolator and the first narrow-bandwidthfilter.

In some embodiments, the first waveguide has a diameter of more than 40microns. In some embodiments, the first optical device includes a firstphotonic-crystal fiber and the first waveguide is a first core of thefirst photonic-crystal device and is capable of operation with a peakpower of about 1 MW or more. In some embodiments, the first core of thefirst photonic-crystal fiber is capable of operation with a peak powerof more than 1 MW. In some embodiments, the first core of the firstphotonic-crystal device is capable of operation with anear-diffraction-limited output beam having M²<1.2. In some embodiments,the first core of the first photonic-crystal device is capable ofoperation with a near-diffraction-limited output beam having M²<1.2 togenerate linearly polarized pulses and a peak power of about 100 kW ormore.

In some embodiments, the present invention provides an apparatus thatincludes a plurality of photonic-crystal optical devices, each includinga first waveguide that has a diameter of at least about 40 microns andmaintains a single transverse mode, wherein the plurality ofphotonic-crystal optical devices are arranged side-by-side and operablesuch that each device emits a pulsed optical beam of a wavelengthdifferent from that of the other devices therein, and at least oneexternal dispersive optical element that receives the emitted opticalbeams and combines them into a single combined beam having M²<2 and peakpower of at least 2 megawatts (MW), the peak power of the combined beambeing at least 50% of the sum of the peak powers in the optical beams.

In some embodiments, the combined beam exhibits peak power of at least 5MW.

In some embodiments, the combined beam exhibits peak power of at least10 MW.

In some embodiments, the combined beam exhibits peak power of at least100 MW.

Some embodiments further include an optical isolator, an optical filter,and a master oscillator subsystem for each of the plurality ofphotonic-crystal optical devices, each master oscillator subsystemoperable to generate a narrow-linewidth, single-frequency seed lasersignal operably coupled to the core of its respective photonic crystaloptical device though the respective optical isolator and the respectiveoptical filter.

In some embodiments, each of the plurality of photonic-crystal opticaldevices includes a photonic-crystal rod (PCR) having rare-earth-dopedcore with a diameter of at least 50 microns and an external diameter ofat least 1 mm such that the rod is therefore thick enough tosubstantially hold its shape when released, and is operable to amplifypulses to obtain an output pulse energy of at least 0.5 mJ, peak powerof at least 1 MW, and beam-quality M² value of less than 1.5.

Some embodiments further include an optical isolator, an optical filter,and a master oscillator subsystem for each of the plurality ofphotonic-crystal optical devices, each master oscillator subsystemoperable to generate a narrow-linewidth, single-frequency seed lasersignal operably coupled to the core of its respective photonic crystaloptical device though the optical isolator and the optical filter, andwherein each of the plurality of photonic-crystal optical devicesincludes a photonic-crystal rod (PCR) having rare-earth-doped core witha diameter of at least 50 microns and an external diameter of at least 1mm such that the rod is therefore thick enough to substantially hold itsshape when released, and is operable to amplify pulses to obtain anoutput pulse energy of at least 0.5 mJ, peak power of at least 1 MW, andbeam-quality M² value of less than 1.5.

In some embodiments, the present invention provides a method thatincludes providing a plurality of photonic-crystal optical devices, eachincluding a first waveguide that has a diameter of at least about 40microns and maintains a single transverse mode, wherein the plurality ofphotonic-crystal optical devices are arranged side-by-side and operablesuch that each device emits a pulsed optical beam of a wavelengthdifferent from that of the other devices therein, and receiving theemitted optical beams and combining the emitted optical beams into asingle combined beam having M²<2 and peak power of at least 2 MW, thepeak power of the combined beam being at least 50% of the sum of thepeak powers in the optical beams.

In some embodiments, the combined beam exhibits peak power of at least 5MW.

In some embodiments, the combined beam exhibits peak power of at least10 MW.

In some embodiments, the combined beam exhibits peak power of at least100 MW.

Some embodiments further include providing an optical isolator, anoptical filter, and a master oscillator subsystem for each of theplurality of photonic-crystal optical devices, generating from eachmaster oscillator subsystem a narrow-linewidth, single-frequency seedlaser signal, and coupling each seed laser signal to the core of itsrespective photonic crystal optical device though the respective opticalisolator and the respective optical filter.

In some embodiments, each of the plurality of photonic-crystal opticaldevices includes a photonic-crystal rod (PCR) having rare-earth-dopedcore with a diameter of at least 50 microns and an external diameter ofat least 1 mm such that the rod is therefore thick enough tosubstantially hold its shape when released, and is operable to amplifypulses to obtain an output pulse energy of at least 0.5 mJ, peak powerof at least 1 MW, and beam-quality M² value of less than 1.5.

Some embodiments further include providing an optical isolator, anoptical filter, and a master oscillator subsystem for each of theplurality of photonic-crystal optical devices, generating from eachmaster oscillator subsystem a narrow-linewidth, single-frequency seedlaser signal, coupling each seed laser signal to the core of itsrespective photonic crystal optical device though the respective opticalisolator and the respective optical filter, wherein each of theplurality of photonic-crystal optical devices includes aphotonic-crystal rod (PCR) having rare-earth-doped core with a diameterof at least 50 microns and an external diameter of at least 1 mm suchthat the rod is therefore thick enough to substantially hold its shapewhen released, and is operable to amplify pulses to obtain an outputpulse energy of at least 0.5 mJ, peak power of at least 1 MW, andbeam-quality M² value of less than 1.5.

In some embodiments, the present invention provides an apparatus thatincludes a plurality of photonic-crystal optical devices, each includinga first waveguide that has a diameter of at least about 40 microns andmaintains a single transverse mode, wherein the plurality ofphotonic-crystal optical devices are arranged side-by-side and operablesuch that each device emits a pulsed optical beam of a wavelengthdifferent from that of the other devices therein, and means forreceiving the emitted optical beams and combining the emitted opticalbeams into a single combined beam having M²<2 and peak power of at least2 MW, the peak power of the combined beam being at least 50% of the sumof the peak powers in the optical beams.

In some embodiments, the combined beam exhibits peak power of at least 5MW.

In some embodiments, the combined beam exhibits peak power of at least10 MW.

In some embodiments, the combined beam exhibits peak power of at least100 MW.

Some embodiments further include an optical isolator, an optical filter,and a master oscillator subsystem for each of the plurality ofphotonic-crystal optical devices, means for generating from each masteroscillator subsystem a narrow-linewidth, single-frequency seed lasersignal, and means for coupling each seed laser signal to the core of itsrespective photonic crystal optical device though the respective opticalisolator and the respective optical filter.

In some embodiments, each of the plurality of photonic-crystal opticaldevices includes a photonic-crystal rod (PCR) having rare-earth-dopedcore with a diameter of at least 50 microns and an external diameter ofat least 1 mm such that the rod is therefore thick enough tosubstantially hold its shape when released, and is operable to amplifypulses to obtain an output pulse energy of at least 0.5 mJ, peak powerof at least 1 MW, and beam-quality M² value of less than 1.5.

Some embodiments further include an optical isolator, an optical filter,and a master oscillator subsystem for each of the plurality ofphotonic-crystal optical devices, means for generating from each masteroscillator subsystem a narrow-linewidth, single-frequency seed lasersignal, means for coupling each seed laser signal to the core of itsrespective photonic crystal optical device though the respective opticalisolator and the respective optical filter, wherein each of theplurality of photonic-crystal optical devices includes aphotonic-crystal rod (PCR) having rare-earth-doped core with a diameterof at least 50 microns and an external diameter of at least 1 mm suchthat the rod is therefore thick enough to substantially hold its shapewhen released, and is operable to amplify pulses to obtain an outputpulse energy of at least 0.5 mJ, peak power of at least 1 MW, andbeam-quality M² value of less than 1.5.

Method and Apparatus for Spectral-Beam Combining of Megawatt-Peak-PowerBeams from Segmented Photonic-Crystal Devices Arranged Side-by-Side

Some embodiments include an apparatus including a plurality of devicesthat each include an optical amplifier having a segmentedphotonic-crystal waveguide that includes a first amplifying segmenthaving a rare-earth-doped photonic-crystal core and a second amplifyingsegment having a rare-earth-doped photonic-crystal core, a masteroscillator operable to generate a seed-laser signal, and a first opticalconnector subassembly operatively coupled between the first segment andthe second segment, wherein each one of the plurality of devices emitsan optical beam of a different wavelength than that of the otherdevices, and the final amplifying segments of photonic-crystalwaveguides are arranged side-by-side, the apparatus further including atleast one external dispersive optical element that acts as aspectral-beam combiner, which receives the emitted optical beams fromeach final amplifying segment of photonic-crystal waveguide and combinesthem into a single combined beam having M²<2 and peak power of 2 MW orhigher, the peak power of the combined beam being at least 50% of thesum of the peak powers in each optical beam. In some such embodiments,the final amplifying segments have an outer diameter of at least 1 mm,and are considered rods, wherein the final photonic-crystal rods arearranged side-by-side, In some embodiments, each respective masteroscillator is operable to emit an optical beam of a different wavelengththan those of the other master oscillators, and each master oscillatorincludes an optical isolator, and a narrow-bandwidth filter, wherein themaster oscillator is operable to generate a narrow-linewidth seed-lasersignal operably coupled to the core of the respective photonic-crystaloptical device through the first optical isolator and the firstnarrow-bandwidth filter.

In some such embodiments, the combined beam exhibits peak power of atleast 3 MW. In some such embodiments, the combined beam exhibits peakpower of at least 4 MW. In some such embodiments, the combined beamexhibits peak power of at least 5 MW. In some such embodiments, thecombined beam exhibits peak power of at least 10 MW. In some suchembodiments, the combined beam exhibits peak power of at least 20 MW. Insome such embodiments, the combined beam exhibits peak power of at least30 MW. In some such embodiments, the combined beam exhibits peak powerof at least 40 MW. In some such embodiments, the combined beam exhibitspeak power of at least 50 MW. In some such embodiments, the combinedbeam exhibits peak power of at least 60 MW. In some such embodiments,the combined beam exhibits peak power of at least 70 MW. In some suchembodiments, the combined beam exhibits peak power of at least 80 MW. Insome such embodiments, the combined beam exhibits peak power of at least90 MW. In some such embodiments, the combined beam exhibits peak powerof at least 100 MW.

In some embodiments, the present invention provides an apparatus thatincludes a first photonic-crystal optical device having a plurality ofphotonic-crystal fibers or rods, each fiber or rod including a core, andan inner pump cladding surrounding its core in order to provide pumplight into the core over a length of the fiber or rod, the deviceincluding a first core that has a diameter of at least about 40 micronsand maintains a single transverse mode. In some embodiments, the opticaldevice includes the plurality of fibers or rods arranged side-by-side,and wherein the cores of the fibers or rods each have a diameter of atleast about 40 microns and each emit a single transverse mode opticalbeam of different wavelength, the apparatus further including one ormore external dispersive optical elements that form a spectral-beamcombiner that receives the emitted optical beams from each core andcombines them into a single beam having M²<2 and peak power of 2 MW orhigher, the peak power of the combined beam being at least 50% of thesum of the peak powers in each optical beam. In some such embodiments,the combined beam exhibits peak power of at least 5 MW. In some suchembodiments, the combined beam exhibits peak power of at least 10 MW. Insome such embodiments, the combined beam exhibits peak power of at least20 MW. In some such embodiments, the combined beam exhibits peak powerof at least 30 MW. In some such embodiments, the combined beam exhibitspeak power of at least 40 MW. In some such embodiments, the combinedbeam exhibits peak power of at least 50 MW. In some such embodiments,the combined beam exhibits peak power of at least 60 MW. In some suchembodiments, the combined beam exhibits peak power of at least 70 MW. Insome such embodiments, the combined beam exhibits peak power of at least80 MW. In some such embodiments, the combined beam exhibits peak powerof at least 90 MW. In some such embodiments, the combined beam exhibitspeak power of at least 100 MW. Some embodiments further include aplurality of master oscillators, each master oscillator emitting anoptical beam of different wavelength and each optical beam being coupledinto one of the spectral-beam combiners.

In some embodiments, the present invention provides an apparatus thatincludes a first photonic-crystal rod (PCR), having rare-earth-dopedcore with a diameter of at least 50 microns and an external diameter ofat least 1 mm such that the rod is therefore thick enough tosubstantially and readily hold its shape when released, wherein thefirst photonic-crystal rod is ribbon-like and further includes aplurality of other cores arranged side-by-side, with their lengthsperpendicular to a straight line transverse to a length of the fiber,and wherein the cores each have a diameter of about 50 microns or moreand each emit a single transverse mode optical beam of differentwavelength, the fiber further including an inner pump claddingsurrounding the core in order to provide pump light into all of thecores over a length of fiber, the apparatus further including one ormore external dispersive optical elements that receive the emittedoptical beams from each core and combine them into a single beam havingM²<2 and peak power of 2 MW or higher, the peak power of the combinedbeam being a significant fraction (50% or higher) of the sum of the peakpowers in each optical beam. In some such embodiments, the combined beamexhibits peak power of at least 5 MW. In some such embodiments, thecombined beam exhibits peak power of at least 10 MW. In some suchembodiments, the combined beam exhibits peak power of at least 20 MW. Insome such embodiments, the combined beam exhibits peak power of at least30 MW. In some such embodiments, the combined beam exhibits peak powerof at least 40 MW. In some such embodiments, the combined beam exhibitspeak power of at least 50 MW. In some such embodiments, the combinedbeam exhibits peak power of at least 60 MW. In some such embodiments,the combined beam exhibits peak power of at least 70 MW. In some suchembodiments, the combined beam exhibits peak power of at least 80 MW. Insome such embodiments, the combined beam exhibits peak power of at least90 MW. In some such embodiments, the combined beam exhibits peak powerof at least 100 MW. Some embodiments further include a plurality ofmaster oscillators, each master oscillator emitting an optical beam ofdifferent wavelength and each optical beam being coupled into one of thespectral-beam combiners.

Monolithic or Ribbon-Like Multi-Core Photonic-Crystal Fibers andAssociated Method

In some embodiments, the present invention provides an apparatus thatincludes a first multi-waveguide ribbon device having an externalthickness of at least 1 mm such that the first multi-waveguide ribbondevice is thick enough to substantially and readily hold its shape whenreleased, and a plurality of substantially parallel cores including afirst rare-earth-doped core with a diameter of at least 50 micronsdefined by photonic-crystal holes and a second rare-earth-doped corewith a diameter of at least 50 microns defined by photonic-crystalholes, the first multi-waveguide ribbon device further including aninner pump cladding surrounding the plurality of cores in order toprovide pump light into the plurality of cores over a length of thefirst multi-waveguide ribbon device.

Some embodiments further include at least one external dispersiveoptical element that receives the emitted optical beams from each of theplurality of cores and combines the beams into a single beam having M²<2and peak power of 2 megawatts (MW) or higher, the peak power of thecombined beam being at least 50% of a sum of the peak powers in eachoptical beam.

In some embodiments, the holes are thermally collapsed along a portionof the first multi-waveguide ribbon device.

In some embodiments, at least one end of the first multi-waveguideribbon device is subjected to a side-tapering process.

Some embodiments further include a first diffraction grating device,wherein output beams of the plurality of cores combined into a singlebeam using the first diffraction grating device.

Some embodiments further include a second multi-waveguide ribbon devicehaving a plurality of substantially parallel cores including a firstrare-earth-doped core defined by photonic-crystal structures and asecond rare-earth-doped core defined by photonic-crystal structures, thesecond photonic-crystal ribbon further including an inner pump claddingsurrounding the plurality of cores in order to provide pump light intothe plurality of cores over a length of the second multi-waveguideribbon device, a first diffraction grating device, wherein output beamsof the plurality of cores from the first multi-waveguide ribbon devicecombined into a single beam using the first diffraction grating device,and a second diffraction grating device, wherein the cores of the secondmulti-waveguide ribbon device are all operated as seed lasers havingdifferent wavelengths that are set using the second diffraction gratingdevice, wherein light from the seed lasers is amplified using the firstmulti-waveguide ribbon device.

Some embodiments further include a second multi-waveguide ribbon devicehaving a plurality of substantially parallel cores including a firstrare-earth-doped core and a second rare-earth-doped core, the secondmulti-waveguide ribbon device further including an inner pump claddingsurrounding the plurality of cores in order to provide pump light intothe plurality of cores over a length of the second ribbon device, afirst diffraction grating device that includes two parallel diffractiongratings, wherein output beams of the plurality of cores from the firstmulti-waveguide ribbon device combined into a single beam using thefirst diffraction grating device, and a second diffraction gratingdevice that includes two parallel diffraction gratings, wherein thecores of the second multi-waveguide ribbon device are all operated asseed lasers having different wavelengths that are set using the seconddiffraction grating device, wherein light from the seed lasers isamplified using the first diffraction grating device.

In some embodiments, the present invention provides a method thatincludes providing a first multi-waveguide ribbon device having anexternal thickness of at least 1 mm such that the first multi-waveguideribbon device is thick enough to substantially and readily hold itsshape when released, and a plurality of substantially parallel coresincluding a first rare-earth-doped core with a diameter of at least 50microns defined by photonic-crystal holes and a second rare-earth-dopedcore with a diameter of at least 50 microns defined by photonic-crystalholes, and injecting pump light into the plurality of cores over alength of the first multi-waveguide ribbon device.

Some embodiments further include spectral-beam combining the beams intoa single beam having M²<2 and peak power of 2 MW or higher, the peakpower of the combined beam being at least 50% of a sum of the peakpowers in each optical beam.

Some embodiments further include thermally collapsing the holes along aportion of the first multi-waveguide ribbon device.

In some embodiments, at least one end of the first multi-waveguideribbon device is subjected to a side-tapering process.

Some embodiments further include providing a first diffraction gratingdevice, and combining output beams of the plurality of cores into asingle beam using the first diffraction grating device.

Some embodiments further include providing a second multi-waveguideribbon device having a plurality of substantially parallel coresincluding a first rare-earth-doped core defined by photonic-crystalstructures and a second rare-earth-doped core defined byphotonic-crystal structures, the second photonic-crystal ribbon furtherincluding an inner pump cladding surrounding the plurality of cores inorder to provide pump light into the plurality of cores over a length ofthe second multi-waveguide ribbon device, spectral-beam combining outputbeams of the plurality of cores from the first multi-waveguide ribbondevice into a single beam using a first diffraction grating device,operating the cores of the second multi-waveguide ribbon device as seedlasers having different wavelengths that are set using a seconddiffraction-grating device, and amplifying light from the seed lasersusing the first multi-waveguide ribbon device.

Some embodiments further include providing a second multi-waveguideribbon device having a plurality of substantially parallel coresincluding a first rare-earth-doped core defined by photonic-crystalstructures and a second rare-earth-doped core defined byphotonic-crystal structures, the second photonic-crystal ribbon furtherincluding an inner pump cladding surrounding the plurality of cores inorder to provide pump light into the plurality of cores over a length ofthe second multi-waveguide ribbon device, spectral-beam combining outputbeams of the plurality of cores from the first multi-waveguide ribbondevice into a single beam using a first diffraction grating device thatincludes two parallel diffraction gratings, operating the cores of thesecond multi-waveguide ribbon device as seed lasers having differentwavelengths that are set using a second diffraction-grating device thatincludes two parallel diffraction gratings, and amplifying light fromthe seed lasers using the first multi-waveguide ribbon device.

In some embodiments, the present invention provides an apparatus thatincludes a first multi-waveguide means for amplifying that substantiallyand readily holds its shape when released, and includes a plurality ofsubstantially parallel cores including a first rare-earth-doped corewith a diameter of at least 50 microns defined by photonic-crystal holesand a second rare-earth-doped core with a diameter of at least 50microns defined by photonic-crystal holes, and means for injecting pumplight into the plurality of cores over a length of the firstmulti-waveguide ribbon device.

Some embodiments further include means for spectral-beam combining thebeams into a single beam having M²<2 and peak power of 2 MW or higher,the peak power of the combined beam being at least 50% of a sum of thepeak powers in each optical beam.

Some embodiments further include means for sealing the holes along aportion of the first multi-waveguide ribbon device.

In some embodiments, at least one end of the first multi-waveguideribbon device is subjected to a side-tapering process.

Some embodiments further include means for combining output beams of theplurality of cores into a single beam.

Some embodiments further include a second multi-waveguide ribbon devicehaving a plurality of substantially parallel cores including a firstrare-earth-doped core defined by photonic-crystal structures and asecond rare-earth-doped core defined by photonic-crystal structures, thesecond photonic-crystal ribbon further including an inner pump claddingsurrounding the plurality of cores in order to provide pump light intothe plurality of cores over a length of the second multi-waveguideribbon device, means for spectral-beam combining output beams of theplurality of cores from the first multi-waveguide ribbon device into asingle beam using a first diffraction grating device, means foroperating the cores of the second multi-waveguide ribbon device as seedlasers having different wavelengths that are set using a seconddiffraction-grating device, means for amplifying light from the seedlasers using the first multi-waveguide ribbon device.

Some embodiments further include a second multi-waveguide ribbon devicehaving a plurality of substantially parallel cores including a firstrare-earth-doped core defined by photonic-crystal structures and asecond rare-earth-doped core defined by photonic-crystal structures, thesecond photonic-crystal ribbon further including an inner pump claddingsurrounding the plurality of cores in order to provide pump light intothe plurality of cores over a length of the second multi-waveguideribbon device, means for spectral-beam combining output beams of theplurality of cores from the first multi-waveguide ribbon device into asingle beam using two parallel means for diffracting, means foroperating the cores of the second multi-waveguide ribbon device as seedlasers having different wavelengths that are set using two parallelmeans for diffracting, and means for amplifying light from the seedlasers using the first multi-waveguide ribbon device.

In some embodiments, the photonic-crystal rod is cut into (or otherwiseformed as) a plurality of segments and the segments are arranged asdescribed in the section above labeled “MULTI-SEGMENTPHOTONIC-CRYSTAL-ROD WAVEGUIDES FOR AMPLIFICATION OF HIGH-POWER PULSEDOPTICAL RADIATION AND ASSOCIATED METHOD.”

In some embodiments, the devices and apparatuses of the presentinvention are operated as the optical source or one of the opticalsources within a device used to process materials, the processing beingany action upon the material that is induced using generated opticalbeams, including, but not limited to, welding, cutting, drilling,annealing, softening, burnishing, abrading, scoring, ablating,vaporizing, chasing, embossing, and melting. In some embodiments, thegenerated optical beams are used in particle accelerators, such asdesktop particle accelerators that use laser beams on thin foils orfocusing cylinders (e.g., such as described by Mike Dunne, “Laser-DrivenParticle Accelerators,” Science, 21 Apr. 2006 312: 374-376).

In some embodiments, the apparatuses are operated as an opticaltransmitter in a device that performs long-range active optical imaging,the list of applicable devices including Light Detector and Ranging(LIDAR) devices that include at least one laser and the wavelength ofone or more optical beams emitted by the apparatus is of 1.0 microns orlonger. In some embodiments, the wavelength of one or more optical beamsemitted by the apparatus is of 1.5 microns or longer. In someembodiments, the combined optical beam from one or more of these devicesis linearly polarized and the degree of polarization is at least 15 dB.In some embodiments, the optical beam emitted by at least one segment islinearly polarized with a degree of polarization being at least 16 dB.In some embodiments, the optical beam emitted by at least one segment islinearly polarized with a degree of polarization being at least 17 dB.In some embodiments, the optical beam emitted by at least one segment islinearly polarized with a degree of polarization being at least 18 dB.In some embodiments, the optical beam emitted by at least one segment islinearly polarized with a degree of polarization being at least 19 dB.In some embodiments, the optical beam emitted by at least one segment islinearly polarized with a degree of polarization being at least 20 dB.Some embodiments further include at least one wavelength-conversionoptical medium, operable to receive a high-peak-power input optical beamhaving a first wavelength and to generate an optical beam having a peakpower of about 100 kW or more and of a different second wavelengththrough wavelength conversion.

In some embodiments, the apparatus is included within, and is beingoperated as an optical transmitter within, a device that performsultra-violet-wavelength laser-induced-fluorescence (UV-LIF) detection,wherein the optical beam of shorter wavelength generated by theapparatus through the wavelength-conversion optical media falls in the200-400-nm wavelength range and its peak power is of 100 kW or more andthe optical beam can be directed towards an airborne cloud carryinginorganic chemicals or organic chemicals or biological compounds, thesechemicals or compounds being able to absorb light at the wavelength ofthe optical beam and release light at a different wavelength byfluorescence and this emitted light can be received and detected by oneor more optical devices that are operated along with the transmitterwithin the UV-LIF device

Multi-Segment Photonic-Crystal-Rod Waveguides Coupled Across aFree-Space Gap and Associated Method

In some embodiments, the present invention provides an apparatus thatincludes a first segmented photonic-crystal-rod (PCR) waveguideconfigured as a plurality of segments with non-contiguousphotonic-crystal rare-earth-doped waveguide cores, the cores having adiameter of at least 40 microns and the segments having an externaldiameter of at least 1 mm, including a second PCR segment and a firstPCR segment having at least one free-space gap therebetween and that areserially encountered along a signal's optical path.

Some embodiments further include a free-space optical subassembly havingan enclosure and a first optical component that forms at least part ofan optical path between the second and first segment, wherein the firstoptical component is laser welded to the enclosure.

In some embodiments, the first optical component is a narrow-bandoptical band-pass filter that is operated also as an optical isolatorfor light at wavelengths other than the signal pulses thatcounter-propagates with respect to the signal pulses.

Some embodiments further include a free-space optical subassembly havingan enclosure and a first optical component that forms at least part ofan optical path between the second and first segment, wherein the firstoptical component is attached to the enclosure using solder.

In some embodiments, the first and second PCR segments are interspacedby a free-space gap and are operated such that the optical beam exitingthe second segment is optically coupled into the first segment using aplurality of optical components.

In some embodiments, the optical beam exiting the second segment isoptically coupled into the first segment using at least a lens and anoptical prism.

In some embodiments, the first and second PCR segments are arranged sideby side such that their cores run parallel to one another, wherein thefirst and second PCR segments are laser welded to one another along atleast a portion of their sides.

Some embodiments further include a substrate, wherein the first andsecond PCR segments are arranged side by side such that their cores runparallel to one another, wherein the first and second PCR segments arelaser welded to the substrate along at least a portion of their sides,and wherein the prism is also laser welded to the substrate.

Some embodiments further include a pump-block subassembly operativelycoupled between the second PCR segment and the first PCR segment,wherein the pump-block subassembly directs pump light into the secondPCR segment in a counter-propagating direction relative to signal light,and directs signal light from the second PCR segment to the first PCRsegment.

Some embodiments further include a pump-block subassembly operativelycoupled between the second PCR segment and the first PCR segment,wherein the pump-block subassembly includes a bandpass optical filterand a dichroic mirror/beamsplitter configured to direct pump light intothe second PCR segment in a counter-propagating direction relative tosignal light, and to direct signal light from the second PCR segment tothe first PCR segment through the bandpass optical filter.

In some embodiments, the core of the first PCR segment is not co-linearwith the core of the second PCR segment.

In some embodiments, at least one of the photonic-crystal rod segmentshas at least one end facet prepared such that the photonic-crystal holesare sealed at the facet and the facet is shaped to form a lens thatcollimates the optical beam propagating outward from the core of thatsegment.

In some embodiments, at least one of the photonic-crystal rod segmentshas at least one end facets prepared such that the cladding holes aresealed at the facet and the facet is shaped to form a lens that focusesinto the segment core an optical beam that is optically coupled into thecore from outside that segment.

In some embodiments, at least one of the second and first PCR segmentsincludes one or more stress regions to create a waveguide that supportsa linearly polarized optical beam having degree of polarization of atleast 15 dB.

In some embodiments, the present invention provides a method thatincludes providing a first segmented photonic-crystal-rod (PCR)waveguide configured as a plurality of segments with non-contiguousphotonic-crystal rare-earth-doped waveguide cores, the cores having adiameter of at least 40 microns and the segments having an externaldiameter of at least 1 mm, including a second PCR segment and a firstPCR segment, and forming an optical signal beam path from the core ofthe second PCR segment to the core of the first PCR segment across atleast one free-space gap.

In some embodiments, the forming of the optical signal beam path furtherincludes providing a free-space optical subassembly having an enclosureand a first optical component that forms at least part of an opticalpath between the second and first segment, and laser welding the firstoptical component to the enclosure.

In some embodiments, the first optical component is a narrow-bandoptical band-pass filter, and the method further includes blockinglight, at wavelengths other than the signal pulses, thatcounter-propagates with respect to the signal pulses.

In some embodiments, the forming of the optical signal beam path furtherincludes providing a free-space optical subassembly having an enclosureand a first optical component that forms at least part of an opticalpath between the second and first segment, and soldering the firstoptical component to the enclosure.

In some embodiments, the forming of the optical signal beam path furtherincludes optically coupling the optical beam exiting the second segmentinto the first segment using a plurality of optical components.

In some embodiments, the forming of the optical signal beam path furtherincludes optically coupling the optical beam exiting the second segmentinto the first segment using at least a lens and an optical prism.

Some embodiments further include providing a substrate, arranging thefirst and second PCR segments side by side such that their cores runparallel to one another, laser welding the first and second PCR segmentsto the substrate along at least a portion of their sides, and laserwelding the prism to the substrate.

Some embodiments further include arranging the first and second PCRsegments side by side such that their cores run parallel to one another,and laser welding the first and second PCR segments to one another alongat least a portion of their sides.

Some embodiments further include providing a pump-block subassembly,

optically coupling signal light from the second PCR segment into thefirst PCR segment through the pump-block subassembly, and directing pumplight into the second PCR segment in a counter-propagating directionrelative to signal light.

Some embodiments further include providing a pump-block subassembly,optically coupling and bandpass filtering signal light from the secondPCR segment into the first PCR segment using a dichroicmirror/beamsplitter and a filter, and directing pump light into thesecond PCR segment in a counter-propagating direction relative to signallight using dichroic mirror/beamsplitter.

Some embodiments further include arranging the core of the first PCRsegment to be not co-linear with the core of the second PCR segment.

Some embodiments further include sealing photonic-crystal holes of atleast one of the photonic-crystal-rod segments at least one end facet,and shaping the facet to form a lens that collimates the optical beampropagating outward from the core of that segment.

Some embodiments further include sealing photonic-crystal holes of atleast one of the PCR segments at least one end facet, and shaping thefacet to form a lens that focuses an optical beam into the segment'score from outside that segment.

Some embodiments further include amplifying a seed signal beam to form alinearly polarized optical signal beam having degree of polarization ofat least 15 dB.

Photonic-Crystal Waveguides with Beam-Expanding Endcaps and AssociatedMethod

In some embodiments, the present invention provides an apparatus thatincludes a photonic-crystal waveguide having lengthwise holessurrounding a core, the waveguide having a first end facet, wherein theholes surrounding the core of the photonic-crystal rod are thermallycollapsed and melted shut over a length of 1 mm or more in a stepwisemanner proceeding from the facet through which the optical beam isemitted, this facet being also angle-polished, such that an optical beampropagating through the region wherein the holes are collapsed canexpand by free-space diffraction). In some embodiments, thephotonic-crystal waveguide is in a photonic-crystal rod havingrare-earth-doped core with a diameter of at least 40 microns and anexternal diameter of at least 1 mm such that the rod is therefore thickenough to substantially and readily hold its shape when released. Insome such embodiments, the core of the PCR has a diameter of at least 50microns. In some such embodiments, the core of the PCR has a diameter ofat least 60 microns. In some such embodiments, the core of the PCR has adiameter of at least 70 microns. In some such embodiments, the core ofthe PCR has a diameter of at least 80 microns. In some such embodiments,the core of the PCR has a diameter of at least 90 microns. In some suchembodiments, the core of the PCR has a diameter of at least 100 microns.In some such embodiments, the core of the PCR has a diameter of at least110 microns. In some such embodiments, the core of the PCR has adiameter of at least 120 microns. In some such embodiments, the core ofthe PCR has a diameter of at least 130 microns. In some suchembodiments, the core of the PCR has a diameter of at least 140 microns.In some such embodiments, the core of the PCR has a diameter of at least150 microns. In some embodiments, the apparatus is operable to generatea peak signal power of at least 500 kW. In some embodiments, the holesare melted shut over a length of 2 mm or more. In some embodiments, theholes are melted shut over a length of 3 mm or more. In someembodiments, the holes are melted shut over a length of 4 mm or more. Insome embodiments, the holes are melted shut over a length of 5 mm ormore.

In some embodiments, the photonic-crystal waveguide is in aphotonic-crystal fiber, and the length over which such holes arethermally collapsed in a stepwise manner being 2 mm or more. In someembodiments, the holes are melted shut over a length of 3 mm or more. Insome embodiments, the holes are melted shut over a length of 4 mm ormore. In some embodiments, the holes are melted shut over a length of 5mm or more.

In some embodiments, the present invention provides an apparatus thatincludes a photonic-crystal waveguide having lengthwise holessurrounding a core, the waveguide having a first end facet, wherein theholes surrounding the core of the photonic-crystal fiber have beenfilled with an compound matching the refractive index of thephotonic-crystal-fiber inner material for a length of 1 mm or more fromthe photonic-crystal-fiber facet.

In some embodiments, the photonic crystal rod is mechanically tapered ina region of several millimeters proceeding from one of its facet,wherein the tapering helps the thermal collapse of the holes.

In some embodiments, the present invention provides an apparatus thatincludes a first photonic-crystal rod (PCR), having rare-earth-dopedcore with a diameter of at least 50 microns and an external diameter ofat least 1 mm such that the rod is therefore thick enough tosubstantially and readily hold its shape when released, wherein thefirst photonic crystal rod is ribbon-like and further includes aplurality of other cores arranged side-by-side generally along astraight line transverse to a length of the fiber, and wherein the coreseach have a diameter of about 50 microns or more and each emit a singletransverse mode optical beam of different wavelength, the fiber furtherincluding an inner pump cladding surrounding the core in order toprovide pump light into all of the cores over a length of fiber, theapparatus further including one or more external dispersive opticalelements that receive the emitted optical beams from each core andcombine them into a single beam having M²<2 and peak power of 2 MW orhigher, the peak power of the combined beam being a significant fraction(50% or higher) of the sum of the peak powers in each optical beam,wherein the holes surrounding the cores of the photonic crystal fiberare thermally collapsed. In some such embodiments, the ends of thephotonic-crystal device are subjected to the above-described taperingprocess.

In some embodiments, the present invention provides for MW-peak-powerradiation at wavelengths of about 1.5 microns or longer produced byPCF/PCR amplifiers.

In some embodiments, the present invention provides the terminationtechniques for photonic band gap fibers, including forming endcaps bycollapsing holes at the end of the fiber and/or fusing a beam-expandingendcap to the end of the fiber.

In some embodiments, the present invention provides for spectral beamcombining (SBC) of two or more beams each having about 1-megawatt (1-MW)or more peak-power beams produced by PCF/PCR amplifiers.

In some embodiments, the present invention provides for collapsing ofholes to make an endcap or the process of successively collapsing fiberto produce longer end caps.

In some embodiments, the present invention provides a master oscillator(MO) with a preamplifier to produce a beam to seed one or more PCF/PCRamplifiers to generate MW peak-power output.

Fiber- or Rod-Based Optical Source Featuring a Large-Core,Rare-Earth-Doped Photonic-Crystal Device for Generation of High-PowerPulsed Radiation and Associated Method

In some embodiments, the present invention provides an apparatus thatincludes a first photonic-crystal optical device that includes a firstwaveguide that has a diameter of at least about 40 microns and maintainsa single transverse mode, is capable of operation with a peak power ofat least 500 kilowatts (kW) and a spectrally narrow signal bandwidth ofless than 20 GHz.

In some embodiments, the first waveguide has a diameter of at least 70microns.

In some embodiments, the first waveguide has a diameter of at least 100microns.

In some embodiments, the first optical device is a firstphotonic-crystal fiber and the first waveguide is a first core of thefirst photonic-crystal device and is capable of operation with a peakpower of at least 4 megawatts (MW).

In some embodiments, the first optical device is a firstphotonic-crystal fiber and the first waveguide is a first core of thefirst photonic-crystal device and is capable of operation with anear-diffraction-limited output beam having M²<1.2.

In some embodiments, the first optical device is a firstphotonic-crystal fiber and the first waveguide is a first core of thefirst photonic-crystal device and is capable of outputting linearlypolarized pulses having a peak power of at least about 100 kW and adegree of polarization of at least 15 dB (wherein the degree ofpolarization is a value of ten times the log (base 10) of the ratio ofoptical power along the polarization axis to the optical power along theorthogonal axis).

In some embodiments, the first optical device is a firstphotonic-crystal fiber and the first waveguide is a first core of thefirst photonic-crystal device and is capable of outputting linearlypolarized pulses having a peak power of at least about 100 kW with anear-diffraction-limited output beam having M²<1.2.

In some embodiments, the first optical device is a firstphotonic-crystal fiber and the first waveguide is a first core of thefirst photonic-crystal device and is capable of outputting linearlypolarized pulses having a peak power of at least about 100 kW with anear-diffraction-limited output beam having M²<1.2 and having a degreeof polarization of at least 15 dB (wherein the degree of polarization isa value of ten times the log (base 10) of the ratio of optical poweralong the polarization axis to the optical power along the orthogonalaxis).

In some embodiments, the first optical device is a firstphotonic-crystal rod having an outer diameter of at least about 1 mm andthe first waveguide is a first core of the first photonic-crystal deviceand has a diameter of at least 50 microns.

Some embodiments further include a second photonic-crystal opticaldevice that includes a core that maintains a single transverse mode,wherein the core is surrounded by a cladding to contain pump light sothe pump light can enter the core over its length, a firstoptical-connector subassembly operatively coupled between the secondphotonic-crystal optical device and the first photonic-crystal opticaldevice, wherein the first optical-connector subassembly directs pumplight into the second photonic-crystal optical device in acounter-propagating direction relative to signal light, and directssignal light from the second photonic-crystal optical device to thefirst photonic-crystal optical device, and a master-oscillator subsystemoperable to obtain a seed laser signal, wherein the master-oscillatorsubsystem includes a first optical isolator and a first narrow-bandwidthband-pass optical filter, and is operable to obtain a narrow-linewidth,single-frequency seed laser signal optically coupled to the core of thesecond photonic-crystal optical device through the first opticalisolator and the first narrow-bandwidth band-pass optical filter.

In some embodiments, the first photonic-crystal optical device includesa second waveguide that maintains a single transverse mode, wherein thefirst waveguide and the second waveguide are each surrounded by acladding to contain pump light so the pump light can enter the coresover their lengths, and the apparatus further includes a firstoptical-connector subassembly operatively coupled between the secondwaveguide and the first waveguide, wherein the first optical-connectorsubassembly directs pump light into the cladding surrounding the secondwaveguide in a counter-propagating direction relative to signal light,and directs signal light from the second waveguide to the firstwaveguide, and a master-oscillator subsystem operable to obtain a seedlaser signal, wherein the master-oscillator subsystem includes a firstoptical isolator and a first narrow-bandwidth band-pass optical filter,and is operable to obtain a narrow-linewidth, single-frequency seedlaser signal optically coupled to the core of the second waveguidethrough the first optical isolator and the first narrow-bandwidthband-pass optical filter.

In some embodiments, the first photonic-crystal optical device includesa plurality of segments including a first segment and a second segment,wherein the first segment includes the first waveguide surrounded by apump cladding and the second segment includes a second waveguidesurrounded by a pump cladding, wherein the first segment and the secondsegment are laser welded to one another side-by-side.

In some embodiments, the present invention provides an apparatus thatincludes an optical waveguide having a diameter of at least about 40microns, and photonic-crystal means for amplifying a pulsed optical beamin the waveguide to a peak power of at least about 1 MW having aspectrally narrow signal bandwidth of less than 10 GHz, and a singletransverse mode.

In some embodiments, the first waveguide has a diameter of at least 70microns.

In some embodiments, the first waveguide has a diameter of at least 100microns.

In some embodiments, the optical waveguide is in a photonic-crystalfiber and the first waveguide is a first core of the photonic-crystalfiber, and wherein the means for amplifying includes means foroutputting a near-diffraction-limited output beam having M²<1.2 andlinearly polarized pulses having a peak power of at least about 100 kW.

In some embodiments, the present invention provides a method thatincludes providing a first photonic-crystal optical device that includesa first waveguide having a diameter of at least about 40 microns, andamplifying a pulsed optical beam in the first waveguide whilemaintaining a single transverse mode to a peak power of at least about 1MW and a spectrally narrow signal bandwidth of less than 20 GHz.

In some embodiments of the method, the first waveguide has a diameter ofat least 70 microns. In some embodiments, the first waveguide has adiameter of at least 100 microns.

In some embodiments, the first optical device is a firstphotonic-crystal fiber and the first waveguide is a first core of thefirst photonic-crystal fiber, and wherein the amplifying includesoutputting a pulsed beam having a peak power of at least 4 MW.

In some embodiments, the first optical device is a firstphotonic-crystal fiber and the first waveguide is a first core of thefirst photonic-crystal fiber, and wherein the amplifying includesoutputting a near-diffraction-limited output beam having M²<1.2.

In some embodiments, the first optical device is a firstphotonic-crystal fiber and the first waveguide is a first core of thefirst photonic-crystal fiber, and wherein the amplifying includesoutputting linearly polarized pulses having a peak power of at leastabout 100 kW.

In some embodiments, the first optical device is a firstphotonic-crystal fiber and the first waveguide is a first core of thefirst photonic-crystal fiber, and wherein the amplifying includesoutputting a near-diffraction-limited output beam having M²<1.2 andlinearly polarized pulses having a peak power of at least about 100 kW.

In some embodiments, the first optical device is a firstphotonic-crystal fiber and the first waveguide is a first core of thefirst photonic-crystal fiber, and wherein the amplifying includesoutputting a near-diffraction-limited output beam having M²<1.2 andlinearly polarized pulses having a peak power of at least about 100 kWand having a degree of polarization of at least 15 dB.

In some embodiments, the first optical device is a firstphotonic-crystal rod having an outer diameter of at least about 1 mm andthe first waveguide is a first core of the first photonic-crystal rodand has a diameter of at least 50 microns.

In some embodiments, the first photonic-crystal optical device includesa second waveguide that maintains a single transverse mode, and themethod further includes generating a single-frequency narrow-linewidthseed laser signal light and directing the seed signal light into thefirst photonic-crystal optical device, directing pump light into thefirst photonic-crystal optical device in a counter-propagating directionrelative to signal light, providing a second photonic-crystal opticaldevice that includes a core that maintains a single transverse mode andis surrounded by a cladding to contain pump light so the pump light canenter the core over its entire length, optically coupling thenarrow-linewidth, single-frequency seed laser signal to the core of thesecond photonic-crystal optical device, directing pump light into thesecond photonic-crystal optical device in a counter-propagatingdirection relative to signal light, amplifying the signal in the secondwaveguide while maintaining a single transverse mode, and directingamplified signal light from the second photonic-crystal optical deviceto the first photonic-crystal optical device.

In some embodiments, the first photonic-crystal optical device includesa second waveguide that maintains a single transverse mode, wherein thefirst waveguide and the second waveguide are each surrounded by claddingto contain pump light so the pump light can enter the cores over theirlengths, and the method further includes generating a narrow-linewidthsingle-frequency seed laser signal, optically coupling thenarrow-linewidth single-frequency seed laser signal into the secondwaveguide, directing pump light into the cladding around the secondwaveguide in a counter-propagating direction relative to signal light,directing signal light from the second waveguide to the first waveguide,amplifying an optical beam in the second waveguide while maintaining asingle transverse mode, and directing signal light from the secondwaveguide to the first waveguide.

In some embodiments, the first photonic-crystal optical device includesa plurality of segments including a first segment and a second segment,wherein the first segment includes the first waveguide surrounded by apump cladding and the second segment includes a second waveguidesurrounded by a pump cladding, and the method further includes laserwelding the first segment and the second segment to one anotherside-by-side.

Photonic-Crystal-Rod Amplifiers for High-Power Pulsed Optical Radiationand Associated Method

In some embodiments, the present invention provides an apparatus thatincludes a photonic-crystal rod (PCR) having rare-earth-doped core witha diameter of at least 40 microns and an external diameter of at least 1mm such that the rod is therefore thick enough to substantially andreadily hold its shape when released, wherein the apparatus is operableto obtain optical pulses from the rod having a peak power of at leastfive hundred kilowatts (500 kW).

In some embodiments, the core of the PCR has a diameter of at least 50microns.

In some embodiments, the core of the PCR has a diameter of at least 70microns.

In some embodiments, the core of the PCR has a diameter of at least 100microns.

In some embodiments, the apparatus is operable to obtain optical pulsesfrom the rod having a peak power of at least two megawatts (2 MW).

In some embodiments, the apparatus is operable to obtain optical pulsesfrom the rod having a beam-quality M² value of less than 1.5.

In some embodiments, the core of the PCR has a diameter of at least 70microns and is Ytterbium (Yb) doped, and is operable to obtain opticalpulses of peak power at least three megawatts (3 MW), a beam-quality M²value of less than 1.5, and a spectral linewidth of less than 13 GHz.

In some embodiments, the core of the PCR has a diameter of at least 100microns and is Ytterbium (Yb) doped, and is operable to generate opticalpulses of peak power at least four megawatts (4 MW), a beam-quality M²value of less than 1.5, and a spectral linewidth of less than 20 GHz.

In some embodiments, the core of the PCR is Ytterbium (Yb) doped, and isoperable to generate linearly polarized optical pulses of peak power atleast one hundred kilowatts (100 kW), and a degree of polarization of atleast 15 dB (wherein the degree of polarization is a value of ten timesthe log (base 10) of the ratio of optical power along the polarizationaxis to the optical power along the orthogonal axis).

Some embodiments further include at least one wavelength-conversionoptical medium operable to receive high-peak-power input radiationhaving a first wavelength from the core of the rod and to generateradiation of a different second wavelength having a peak power of atleast about 100 kW through wavelength conversion.

In some embodiments, the present invention provides a method thatincludes providing a photonic-crystal rod (PCR) having rare-earth-dopedcore with a diameter of at least 50 microns and an external diameter ofat least 1 mm, and amplifying optical pulses with the rod to generatepulses having a peak power of at least five hundred kilowatts (500 kW).

In some embodiments, the core of the PCR has a diameter of at least 50microns.

In some embodiments, the core of the PCR has a diameter of at least 70microns.

In some embodiments, the core of the PCR has a diameter of at least 100microns.

In some embodiments, the amplifying generates optical pulses from therod having a peak power of at least two megawatts (2 MW).

In some embodiments, the amplifying generates optical pulses from therod having a beam-quality M² value of less than 1.5.

In some embodiments, the amplifying generates optical pulses of peakpower at least three megawatts (3 MW), a beam-quality M² value of lessthan 1.5, and a spectral linewidth of less than 13 GHz.

In some embodiments, the amplifying generates optical pulses of peakpower at least four megawatts (4 MW), a beam-quality M² value of lessthan 1.5, and a spectral linewidth of less than 20 GHz.

In some embodiments, the core of the PCR is Ytterbium (Yb) doped, andwherein the amplifying generates linearly polarized optical pulses ofpeak power at least one hundred kilowatts (100 kW), and a degree ofpolarization of at least 15 dB (wherein the degree of polarization is avalue of ten times the log (base 10) of the ratio of optical power alongthe polarization axis to the optical power along the orthogonal axis).

Some embodiments further include receiving high-peak-power inputradiation having a first wavelength from the PCR, and performingwavelength-conversion to generate radiation of a different secondwavelength and having a peak power of at least about 100 kW.

In some embodiments, the present invention provides an apparatus thatincludes a photonic-crystal rod (PCR) having rare-earth-doped core witha diameter of at least 50 microns and an external diameter of at least 1mm, and means for amplifying optical pulses with the rod to generatepulses having a peak power of at least five hundred kilowatts (500 kW).

In some embodiments, the means for amplifying generates optical pulsesfrom the rod having a peak power of at least two megawatts (2 MW).

In some embodiments, the means for amplifying generates optical pulsesof peak power at least four megawatts (4 MW), a beam-quality M² value ofless than 1.5, and a spectral linewidth of less than 20 GHz.

In some embodiments, the core of the PCR is Ytterbium (Yb) doped, andthe means for amplifying generates linearly polarized optical pulses ofpeak power at least one hundred kilowatts (100 kW), and a degree ofpolarization of at least 15 dB (wherein the degree of polarization is avalue of ten times the log (base 10) of the ratio of optical power alongthe polarization axis to the optical power along the orthogonal axis).

Some embodiments further include means for performingwavelength-conversion to generate radiation of a different secondwavelength and having a peak power of at least about 100 kW.

Multi-Stage Optical Amplifier Having Photonic-Crystal-Rod Waveguides andNon-Photonic-Crystal Optical Fiber Interconnects and Associated Method

In some embodiments, the present invention provides an apparatus thatincludes an optical amplifier having a first photonic-crystal rod (PCR)that has a rare-earth-doped photonic-crystal core with a diameter of atleast 40 microns and a cladding having an outer diameter of at least 1mm, a master oscillator operable to generate a seed laser signaloperatively coupled to the optical amplifier, and a first opticalcomponent having a solid-body fiber that has a signal waveguide directlyoptically coupled to the core of the first PCR without a free-space gap,wherein the solid-body fiber does not have a photonic-crystal structurein the optical signal path.

In some embodiments, the core of the first PCR maintains a singletransverse mode, is capable of operation with a peak power of at least500 kW and a spectrally narrow signal bandwidth of less than 20 GHz.

In some embodiments, the core of the first PCR outputs linearlypolarized pulses having a peak power of at least about 100 kW and adegree of polarization of at least 15 dB (wherein the degree ofpolarization is a value of ten times the log (base 10) of the ratio ofoptical power along the polarization axis to the optical power along theorthogonal axis).

In some embodiments, the core of the first PCR has a diameter of atleast 70 microns.

In some embodiments, the optical amplifier further includes a second PCRthat has a rare-earth-doped photonic-crystal core with a diameter of atleast 40 microns and a cladding having an outer diameter of at least 1mm, and wherein the signal waveguide of the first optical component isdirectly optically coupled to the core of the second PCR without afree-space gap.

Some embodiments further include a substrate, wherein the first PCR andthe second PCR are arranged side by side such that their cores runparallel to one another, wherein the first PCR and the second PCR arelaser welded to the substrate along at least a portion of their sides.

Some embodiments further include a pump-block subassembly operativelycoupled to the first PCR, wherein the pump-block subassembly directspump light into the first PCR in a counter-propagating directionrelative to signal light, and directs signal light out from the firstPCR.

Some embodiments further include a pump-block subassembly operativelycoupled between the second PCR and the first PCR, wherein the pump-blocksubassembly includes a bandpass optical filter and a dichroicmirror/beamsplitter configured to direct pump light into the second PCRin a counter-propagating direction relative to signal light, and todirect signal light from the second PCR to the first PCR through thebandpass optical filter.

In some embodiments, the present invention provides an apparatus thatincludes an optical amplifier having a first photonic-crystal rod (PCR)that has a rare-earth-doped photonic-crystal core with a diameter of atleast 40 microns and a cladding having an outer diameter of at least 1mm, and non-photonic-crystal means for forming an optical signal pathconnected to the core of the first PCR without a free-space gap.

Some embodiments further include means for amplifying optical pulses inthe first PCR to a peak power of at least 500 kW and a spectrally narrowsignal bandwidth of less than 20 GHz.

Some embodiments further include means for amplifying optical pulses inthe first PCR to a peak power of at least about 100 kW and a degree ofpolarization of at least 15 dB (wherein the degree of polarization is avalue of ten times the log (base 10) of the ratio of optical power alongthe polarization axis to the optical power along the orthogonal axis).

In some embodiments, the provided optical amplifier further includes asecond PCR that has a rare-earth-doped photonic-crystal core with adiameter of at least 40 microns and a cladding having an outer diameterof at least 1 mm, wherein the non-photonic-crystal means is alsoconnected to the core of the second PCR without a free-space gap.

In some embodiments, the present invention provides a method thatincludes providing an optical amplifier having a first photonic-crystalrod (PCR) that has a rare-earth-doped photonic-crystal core with adiameter of at least 40 microns and a cladding having an outer diameterof at least 1 mm, providing a master oscillator, providing a solid-bodyfiber that has a signal waveguide, wherein the solid-body fiber does nothave a photonic-crystal structure in the optical signal path, generatinga seed laser signal with the master oscillator, optically coupling theseed laser signal to the optical amplifier, and directly opticallycoupling the signal waveguide of the solid-body fiber to the core of thefirst PCR without a free-space gap.

Some embodiments further include amplifying optical pulses in the firstPCR to a peak power of at least 500 kW and a spectrally narrow signalbandwidth of less than 20 GHz.

Some embodiments further include amplifying optical pulses in the firstPCR to a peak power of at least about 100 kW and a degree ofpolarization of at least 15 dB (wherein the degree of polarization is avalue of ten times the log (base 10) of the ratio of optical power alongthe polarization axis to the optical power along the orthogonal axis).

In some embodiments, the core of the first PCR has a diameter of atleast 70 microns.

In some embodiments, the provided optical amplifier further includes asecond PCR that has a rare-earth-doped photonic-crystal core with adiameter of at least 40 microns and a cladding having an outer diameterof at least 1 mm, the method further including directly opticallycoupling the signal waveguide of the solid-body fiber to the core of thesecond PCR without a free-space gap.

Some embodiments further include providing a substrate, arranging thefirst PCR and the second PCR side by side such that their cores runparallel to one another, and laser welding the first PCR and the secondPCR to the substrate along at least a portion of their sides.

Some embodiments further include providing a pump-block subassembly,directing pump light with the pump-block subassembly into the first PCRin a counter-propagating direction relative to signal light, anddirecting signal light out from the first PCR.

Some embodiments further include providing a pump-block subassemblyoperatively coupled between the second PCR and the first PCR, whereinthe pump-block subassembly includes a bandpass optical filter and adichroic mirror/beamsplitter, optically coupling and bandpass filteringsignal light from the second PCR into the first PCR using a dichroicmirror/beamsplitter and a filter, and directing pump light into thesecond PCR in a counter-propagating direction relative to signal lightusing dichroic mirror/beamsplitter.

Photonic-Crystal-Rod Optical Amplifier with Sealed-Hole Endcap andAssociated Method

In some embodiments, the present invention provides an apparatus thatincludes a photonic-crystal optical device having a core surrounded by aplurality of lengthwise holes that extend from at or near a first end ofthe device to at or near a second end of the device, wherein the holesare sealed at the second end of the device to form a beam-expandingendcap.

In some embodiments, the device is a first photonic-crystal rod (PCR)having an outer diameter of at least 1 mm.

In some embodiments, the holes are sealed to a distance of at least 1 mmfrom the second end of the first PCR device.

In some embodiments, the holes are collapsed to a distance of at least 1mm from the second end of the first PCR device.

In some embodiments, the holes are filled with an index-matchingmaterial to a distance of at least 1 mm from the second end of the firstPCR device.

In some embodiments, the endcap of the first PCR is formed to a diametersmaller than a diameter of the first PCR away from the endcap, and afacet is formed at an end of the endcap of the first PCR.

Some embodiments further include a second photonic-crystal rod having anendcap that is formed to a diameter smaller than a diameter of thesecond PCR away from the endcap, and having a facet formed on the endcapof the second PCR, wherein the endcap of the first PCR is placedside-by-side to the endcap of the second PCR such that the end facet ofthe first PCR and the end facet of the second PCR are placed at acenter-to-center distance closer than the diameter of the first PCR awayfrom the endcap of the first PCR.

Some embodiments further include a solid-glass endcap that islaser-welded to the second end of the first PCR device.

Some embodiments further include a solid-glass endcap that is fused tothe second end of the first PCR device.

In some embodiments, the present invention provides an apparatus thatincludes a photonic-crystal optical device having a core surrounded by aplurality of lengthwise holes that extend from a first end of the deviceto a second end of the device, and means for sealing the holes for adistance from the second end of the device to form a beam-expandingendcap.

In some embodiments, the means for sealing includes a fiber-splicingdevice

In some embodiments, the device is a first photonic-crystal rod (PCR)having an outer diameter of at least 1 mm, and the means for sealingincludes means for melting the holes shut to a distance of at least 1 mmfrom the second end of the first PCR device, and means for polishing aoptical exit facet on the sealed end.

In some embodiments, the device is a first photonic-crystal rod (PCR)having an outer diameter of at least 1 mm, and the means for sealingincludes means for temporarily sealing the holes at the second end withan epoxy, means for tapering a diameter of the second end, means forremoving the epoxy seal, means for melting the holes shut at the secondend, and means for faceting the second end.

In some embodiments, the means for sealing includes means forlaser-welding a solid-glass endcap to the second end of the first PCRdevice.

In some embodiments, the present invention provides a method thatincludes providing a photonic-crystal optical device having a coresurrounded by a plurality of lengthwise holes that extend from a firstend of the device to a second end of the device, and sealing the holesfor a distance from the second end of the device to form abeam-expanding endcap.

In some embodiments, the sealing includes melting the holes shut at thesecond end to form a solid-glass beam-expanding endcap.

In some embodiments, the device is a first photonic-crystal rod (PCR)having an outer diameter of at least 1 mm, and the sealing includesmelting the holes shut to a distance of at least 1 mm from the secondend of the first PCR device, and polishing a optical exit facet on thesealed end.

In some embodiments, the device is a first photonic-crystal rod (PCR)having an outer diameter of at least 1 mm, and the sealing includesaffixing a temporary seal to the holes at the second end, reducing adiameter of the second end, removing the temporary seal, melting theholes shut at the second end, and faceting the second end.

In some embodiments, the device is a first photonic-crystal rod (PCR)having an outer diameter of at least 1 mm, and the sealing includestemporarily sealing the holes at the second end with an epoxy, taperinga diameter of the second end, removing the epoxy seal, melting the holesshut at the second end, and faceting the second end.

In some embodiments, the sealing includes sealing the holes to adistance of at least 1 mm from the second end of the first PCR device.

In some embodiments, the sealing includes collapsing the holes to adistance of at least 1 mm from the second end of the first PCR device.

In some embodiments, the sealing includes laser-welding a solid-glassendcap to the second end of the first PCR device.

In some embodiments, the sealing includes fusing a solid-glass endcap tothe second end of the first PCR device.

In some embodiments, the sealing includes filling the holes with anindex-matching material to a distance of at least 1 mm from the secondend of the first PCR device.

Monolithic Pump Block for Optical Amplifiers and Associated Method

In some embodiments, the present invention provides an apparatus thatincludes an integral pump block having a signal input coupler, a signaloutput coupler, and a pump input coupler, wherein the pump input coupleris optically coupled to direct pump light into the signal input couplerin a counter-propagating direction relative to signal light, and thesignal input coupler is optically coupled to direct signal light intothe signal output coupler.

In some embodiments, the pump block further includes a glass substrateand a plurality of optical components that are laser welded to thesubstrate.

In some embodiments, the pump block further includes a substrate, abandpass optical filter and a dichroic mirror/beamsplitter configured todirect pump light into the signal input coupler in a counter-propagatingdirection relative to signal light, and to direct signal light from thesignal input coupler to the signal output coupler through the bandpassoptical filter, wherein the bandpass filter and the dichroicmirror/beamsplitter are laser welded to the substrate.

In some embodiments, the pump block further includes a first lensconfigured to collimate signal input light from the signal input couplertoward the dichroic mirror/beamsplitter and bandpass filter and to focuspump light from the dichroic mirror/beamsplitter toward the signal inputcoupler, a second lens configured to collimate pump light from the pumpinput coupler toward the dichroic mirror/beamsplitter, and a third lensconfigured to focus pump light from the dichroic mirror/beamsplitter andbandpass filter toward the signal output coupler.

In some embodiments, the first, second, and third lens are laser weldedto the substrate.

In some embodiments, the pump block further includes a substrate and adichroic mirror/beamsplitter configured to direct pump light into thesignal input coupler in a counter-propagating direction relative tosignal light, and to direct signal light from the signal input couplerto the signal output coupler, wherein the dichroic mirror/beamsplitteris laser welded to the substrate.

In some embodiments, the pump block further includes a first lensconfigured to collimate signal input light from the signal input couplertoward the dichroic mirror/beamsplitter and to focus pump light from thedichroic mirror/beamsplitter toward the signal input coupler, a secondlens configured to collimate pump light from the pump input couplertoward the dichroic mirror/beamsplitter, and a third lens configured tofocus pump light from the dichroic mirror/beamsplitter toward the signaloutput coupler.

In some embodiments, the pump block further includes a substrate and aplurality of optical components that are affixed to the substrate usingsolder.

In some embodiments, the pump block further includes a substrate, abandpass optical filter and a dichroic mirror/beamsplitter configured todirect pump light into the signal input coupler in a counter-propagatingdirection relative to signal light, and to direct signal light from thesignal input coupler to the signal output coupler through the bandpassoptical filter, wherein the bandpass filter and the dichroicmirror/beamsplitter are affixed to the substrate using solder.

In some embodiments, the pump block further includes a first lensconfigured to collimate signal input light from the signal input couplertoward the dichroic mirror/beamsplitter and bandpass filter and to focuspump light from the dichroic mirror/beamsplitter toward the signal inputcoupler, a second lens configured to collimate pump light from the pumpinput coupler toward the dichroic mirror/beamsplitter, and a third lensconfigured to focus pump light from the dichroic mirror/beamsplitter andbandpass filter toward the signal output coupler, wherein the first,second, and third lens are affixed to the substrate using solder.

In some embodiments, the pump block further includes a substrate, adichroic mirror/beamsplitter configured to direct pump light into thesignal input coupler in a counter-propagating direction relative tosignal light, and to direct signal light from the signal input couplerto the signal output coupler, a first lens configured to collimatesignal input light from the signal input coupler toward the dichroicmirror/beamsplitter and to focus pump light from the dichroicmirror/beamsplitter toward the signal input coupler, a second lensconfigured to collimate pump light from the pump input coupler towardthe dichroic mirror/beamsplitter, and a third lens configured to focuspump light from the dichroic mirror/beamsplitter toward the signaloutput coupler, wherein at least one of the dichroicmirror/beamsplitter, the first lens, the second lens, and the third lensare affixed to the substrate using solder.

In some embodiments, the present invention provides an apparatus thatincludes an integral pump block having a signal input coupler, a signaloutput coupler, and a pump input coupler, wherein the pump input coupleris optically coupled to direct pump light into the signal input couplerin a counter-propagating direction relative to signal light, and thesignal input coupler is optically coupled to direct signal light intothe signal output coupler.

In some embodiments, the present invention provides a method thatincludes integrally forming a pump block having a signal input coupler,a signal output coupler, and a pump input coupler, including opticallycoupling the pump input coupler to direct pump light into the signalinput coupler in a counter-propagating direction relative to signallight, and optically coupling the signal input coupler to direct signallight into the signal output coupler.

In some embodiments, the integrally forming includes providing a glasssubstrate and laser welding a plurality of optical components to thesubstrate.

In some embodiments, the integrally forming includes providing a glasssubstrate, a bandpass optical filter, and a dichroicmirror/beamsplitter, and laser welding the bandpass optical filter andthe dichroic mirror/beamsplitter to the substrate so as to direct pumplight into the signal input coupler in a counter-propagating directionrelative to signal light, and to direct signal light from the signalinput coupler to the signal output coupler through the bandpass opticalfilter.

In some embodiments, the integrally forming includes providing a firstlens, a second lens, and a third lens, configuring the first lens tocollimate signal input light from the signal input coupler toward thedichroic mirror/beamsplitter and bandpass filter and to focus pump lightfrom the dichroic mirror/beamsplitter toward the signal input coupler,configuring the second lens to collimate pump light from the pump inputcoupler toward the dichroic mirror/beamsplitter, and configuring thethird lens to focus pump light from the dichroic mirror/beamsplitter andbandpass filter toward the signal output coupler. In some embodiments,the integrally forming includes laser welding the first, second, andthird lens to the substrate.

In some embodiments, the integrally forming includes providing a glasssubstrate and a dichroic mirror/beamsplitter, and laser welding thedichroic mirror/beamsplitter to the substrate so as to direct pump lightinto the signal input coupler in a counter-propagating directionrelative to signal light, and to direct signal light from the signalinput coupler to the signal output coupler.

In some embodiments, the integrally forming includes providing a firstlens, a second lens, and a third lens, configuring the first lens tocollimate signal input light from the signal input coupler toward thedichroic mirror/beamsplitter and to focus pump light from the dichroicmirror/beamsplitter toward the signal input coupler, configuring thesecond lens to collimate pump light from the pump input coupler towardthe dichroic mirror/beamsplitter, and configuring the third lens tofocus pump light from the dichroic mirror/beamsplitter toward the signaloutput coupler.

In some embodiments, the integrally forming includes laser welding thefirst, second, and third lens to the substrate. In some embodiments, theintegrally forming includes providing a glass substrate and laserwelding a plurality of optical components to the substrate.

In some embodiments, the integrally forming includes providing asubstrate, a bandpass optical filter, and a dichroicmirror/beamsplitter, and affixing the bandpass optical filter and thedichroic mirror/beamsplitter to the substrate using solder so as todirect pump light into the signal input coupler in a counter-propagatingdirection relative to signal light, and to direct signal light from thesignal input coupler to the signal output coupler through the bandpassoptical filter.

In some embodiments, the integrally forming includes providing a firstlens, a second lens, and a third lens, configuring the first lens tocollimate signal input light from the signal input coupler toward thedichroic mirror/beamsplitter and bandpass filter and to focus pump lightfrom the dichroic mirror/beamsplitter toward the signal input coupler,configuring the second lens to collimate pump light from the pump inputcoupler toward the dichroic mirror/beamsplitter, and configuring thethird lens to focus pump light from the dichroic mirror/beamsplitter andbandpass filter toward the signal output coupler, wherein the first,second, and third lens are affixed to the substrate using solder.

In some embodiments, the integrally forming includes providing a firstlens, a second lens, a third lens, a substrate, and a dichroicmirror/beamsplitter, configuring the dichroic mirror/beamsplitterconfigured to direct pump light into the signal input coupler in acounter-propagating direction relative to signal light, and to directsignal light from the signal input coupler to the signal output coupler,configuring the first lens to collimate signal input light from thesignal input coupler toward the dichroic mirror/beamsplitter and tofocus pump light from the dichroic mirror/beamsplitter toward the signalinput coupler, configuring the second lens to collimate pump light fromthe pump input coupler toward the dichroic mirror/beamsplitter, andconfiguring the third lens to focus pump light from the dichroicmirror/beamsplitter toward the signal output coupler, wherein at leastone of the configuring of the dichroic mirror/beamsplitter, the firstlens, the second lens, and the third lens include affixing to thesubstrate using solder.

It is to be understood that the above description is intended to beillustrative, and not restrictive. Although numerous characteristics andadvantages of various embodiments as described herein have been setforth in the foregoing description, together with details of thestructure and function of various embodiments, many other embodimentsand changes to details will be apparent to those of skill in the artupon reviewing the above description. The scope of the invention shouldbe, therefore, determined with reference to the appended claims, alongwith the full scope of equivalents to which such claims are entitled. Inthe appended claims, the terms “including” and “in which” are used asthe plain-English equivalents of the respective terms “comprising” and“wherein,” respectively. Moreover, the terms “first,” “second,” and“third,” etc., are used merely as labels, and are not intended to imposenumerical requirements on their objects.

What is claimed is:
 1. A system for combining a plurality of laserbeamlets to form a single collimated output beam using spectral beamcombination, the system comprising: a plurality of laser sourcesemitting a plurality of beamlets, wherein each one of the plurality ofbeamlets has a different wavelength; a plurality of lenses arranged tocollimate each one of the plurality of beamlets; a spatial chirperconfigured to apply a spatial chirp to each one of the collimatedplurality of beamlets such that spectral components of the plurality ofbeamlets are transversely dispersed; and a dispersive beam combinerconfigured to remove the spatial chirp imposed by the spatial chirperand to combine the plurality of beamlets into a single collimated outputbeam.
 2. The system of claim 1, wherein the spatial chirper includes aplurality of lens-grating pairs, wherein each lens-grating pair isconfigured to apply a spatial chirp to a corresponding one of thecollimated beamlets.
 3. The system of claim 2, wherein the plurality oflens-grating pairs are arranged in a one-dimensional linear array. 4.The system of claim 1, wherein the spatial chirper includes a pluralityof diffraction gratings, wherein each diffraction grating is arranged toapply a spatial chirp to a corresponding one of the plurality ofcollimated beamlets.
 5. The system of claim 1, wherein the spatialchirper and the beam combiner each include a reflective grating with adielectric coating.
 6. The system of claim 1, wherein the plurality oflaser sources includes a laser fiber array.
 7. The system of claim 6,wherein the laser fiber array includes about one hundred (100) laserfibers arranged in a one-dimensional linear array.
 8. The system ofclaim 1, wherein each one of the plurality of laser sources has abandwidth of about 1 nm.
 9. The system of claim 1, wherein the singlecollimated output beam has an output power of at least 10,000 W.
 10. Thesystem of claim 1, wherein the spatial chirper includes one or moreoptical elements further comprising an active positioning controlsubsystem operably connected to one or more of the optical elements. 11.The system of claim 1, further comprising an astigmatic focusing elementconfigured to reshape the output beam such that a height-to-width ratioof the output beam is substantially one.
 12. The system of claim 1,wherein the plurality of laser sources includes a plurality ofphotonic-crystal-fiber (PCF) amplifiers.
 13. A method for combining aplurality of laser beamlets to form a single collimated output beamusing spectral beam combination, comprising: emitting a plurality oflaser beamlets from a plurality of laser sources, wherein each one ofthe plurality of beamlets has a different wavelength; dispersivelyspatially chirping to each one of the plurality of beamlets to produce atransverse distribution of spectral beam components; and dispersivelycombining the plurality of spatially-chirped beamlets into a singlecollimated output beam to remove the spatial chirp imposed by thespatially chirping operation.
 14. The method of claim 13, wherein thedispersively spatially chirping includes transmitting each one of theplurality of beamlets to a corresponding one of the plurality oflens-grating pairs.
 15. The method of claim 13, wherein the dispersivelyspatially chirping includes transmitting each one of the plurality ofbeamlets to a corresponding one of the plurality of grating pairsincluding first and second gratings.
 16. The method of claim 13, furthercomprising actively controlling the position of at least one opticalelement of the spatial chirper using an active positioning controlsubsystem.
 17. The method of claim 13, further comprising: providing anastigmatic focusing element; and reshaping the output beam using thefocusing element such that a height-to-width ratio of the output beam issubstantially one.
 18. The method of claim 13, wherein each one of theplurality of laser sources has a bandwidth of about 1 nm.
 19. A systemfor combining a plurality of laser beamlets to form a single collimatedoutput beam using spectral beam combination, the system comprising: aplurality of laser sources emitting a plurality of beamlets, whereineach one of the plurality of beamlets has a different wavelength; aplurality of lenses arranged to collimate each one of the plurality ofbeamlets; a spatial chirper configured to apply a spatial chirp to eachone of the collimated plurality of beamlets such that spectralcomponents of the plurality of beamlets are transversely dispersed,wherein the spatial chirper includes a grating pair configured to applya spatial chirp to each one of the plurality of collimated beamlets. 20.The system of claim 19, further comprising an astigmatic focusingelement configured to reshape the output beam such that aheight-to-width ratio of the output beam is substantially one.